Embedders can now notify shell during startup about the various displays and their corresponding settings.
Adds a notion of Display update type which can later include chages to displays during runtime such as addition / removal / reconfiguration of displays.

We also remove the responsibility of providing the refresh rate from `vsync_waiter` to `DisplayManager`.
Rewires existing platform implementations of the said API to use `Shell::OnDisplayUpdate` to notify the display manager of the startup configuration.

DisplayManager is also thread-safe to account for rasterizer and UI thread accesses.

Added a new `FlutterEngineAOTData` argument to `FlutterProjectArgs`. Embedders can instantiate and destroy this object via the new `FlutterEngineCreateAOTData` and `FlutterEngineCollectAOTData` methods provided.

If an embedder provides more than one source of AOT data to `FlutterEngineInitialize` or `FlutterEngineRun` (e.g. snapshots as well as `FlutterEngineAOTData`), the engine will error out.

Resolves: https://github.com/flutter/flutter/issues/50778

`FlutterEnginePostCallbackOnAllNativeThreads` schedule a callback to be run on
all engine managed threads. The engine will attempt to service this callback the
next time the message loops for each managed thread is idle. Since the engine
manages the entire lifecycle of multiple threads, there is no opportunity for
the embedders to finely tune the priorities of threads directly, or, perform
other thread specific configuration (for example, setting thread names for
tracing). This callback gives embedders a chance to affect such tuning.

Fixes flutter/flutter#49551
Fixes b/143774406
Fixes b/148278215
Fixes b/148278931

This is a non-breaking addition to the stable Flutter Embedder API.

This is a non-breaking addition to the stable Flutter Embedder API and exposes a
subset of the functionality provided by Dart_PostCObject API in a stable and
tested manner to custom embedder implementations.

Send port acquisition can currently be done as described in the unit-test but
there may be opportunities to extend this API in the future to access ports more
easily or create ports from the native side.

The following capabilities of the the Dart_PostCObject API are explicitly NOT
exposed:
* Object arrays: This allows callers to create complex object graphs but only
  using the primitives specified in the native API. I could find no current use
  case for this and would have made the implementation a lot more complex. This
  is something we can add in the future if necessary however.
* Capabilities and ports: Again no use cases and I honestly I didn’t understand
  how to use capabilities. If needed, these can be added at a later point by
  appending to the union.

Fixes https://github.com/flutter/flutter/issues/46624
Fixes b/145982720

This allows custom compositors to affect scene builder modifications made
to the platform view.

Fixes https://github.com/flutter/flutter/issues/44211
Fixes b/143612326

This exposes platform_view_id on the embedder API's FlutterSemanticNode.

In bd0f9085 (#8055), platformViewId was
added to SemanticsNode. This field is non-zero when the SemanticsNode
represents a platform view and is typically used by embedders as a means
of identifying locations where a platform view's 'native' accessibility
tree should be injected into the platform-specific accessibility tree
constructed by the embedder.

Due to the intended use of this field, the Flutter framework is meant to
enforce that this node has a child count of zero.

For embedder code that is configured for both AOT and JIT mode Dart execution
based on the Flutter engine being linked to, this runtime check may be used to
appropriately configure the `FlutterProjectArgs`. In JIT mode execution, the
kernel snapshots must be present in the Flutter assets directory specified in
the `FlutterProjectArgs`. For AOT execution, the fields `vm_snapshot_data`,
`vm_snapshot_instructions`, `isolate_snapshot_data` and
`isolate_snapshot_instructions` (along with their size fields) must be specified
in `FlutterProjectArgs`.

This was previously only possible by sending an undocumented payload over an engine managed channel.

Embedders may use this to specify a thread whose event loop is managed by them
instead of the engine. In addition, specifying the same task runner for both
the platform and render task runners allows embedders to effectively perform
GPU rendering operations on the platform thread.

To affect this change, the following non breaking changes to the API have been
made:

* The `FlutterCustomTaskRunners` struct now has a new field `render_task_runner`
  for the specification of a custom render task runner.
* The `FlutterTaskRunnerDescription` has a new field `identifier`. Embedders
  must supply a unique identifier for each task runner they specify. In
  addition, when describing multiple task runners that run their tasks on the
  same thread, their identifiers must match.
* The embedder may need to process tasks during `FlutterEngineRun` and
  `FlutterEngineShutdown`. However, the embedder doesn't have the Flutter engine
  handle before `FlutterEngineRun` and is supposed to relinquish handle right
  before `FlutterEngineShutdown`. Since the embedder needs the Flutter engine
  handle to service tasks on other threads while these calls are underway,
  there exist opportunities for deadlock. To work around this scenario, three
  new calls have been added that allow more deliberate management of the Flutter
  engine instance.
  * `FlutterEngineRun` can be replaced with `FlutterEngineInitialize` and
    `FlutterEngineRunInitialized`. The embedder can obtain a handle to the
    engine after the first call but the engine will not post any tasks to custom
    task runners specified by the embedder till the
    `FlutterEngineRunInitialized` call. Embedders can guard the Flutter engine
    handle behind a mutex for safe task runner interop.
  * `FlutterEngineShutdown` can be preceded by the `FlutterEngineDeinitialize`
    call. After this call the Flutter engine will no longer post tasks onto
    embedder managed task runners. It is still embedder responsibility to
    collect the Flutter engine handle via `FlutterEngineShutdown`.
* To maintain backwards compatibility with the old APIs, `FlutterEngineRun` is
  now just a convenience for `FlutterEngineInitialize` and
  `FlutterEngineRunInitilaized`. `FlutterEngineShutdown` now implicitly calls
  `FlutterEngineDeinitialize` as well. This allows existing users who don't care
  are custom task runner interop to keep using the old APIs.
* Adds complete test coverage for both old and new paths.

Fixes https://github.com/flutter/flutter/issues/42460
Prerequisite for https://github.com/flutter/flutter/issues/17579

For https://github.com/flutter/flutter/issues/40686

Unit tests added:
- CacheSkSLWorks
- VisitFilesCanBeCalledTwice
- CanListFilesRecursively

This reverts commit 974ca210.

Adding missing flag for IsFocusable that I missed in #12618

* Add windows font change logic

* update

* fix comment

Account for root surface transformation on the surfaces managed by the external view embedder. (#11384)

The earlier design speculated that embedders could affect the same
transformations on the layers post engine compositor presentation but before
final composition.

However, the linked issue points out that this design is not suitable for use
with hardware overlay planes. When rendering to the same, to affect the
transformation before composition, embedders would have to render to an
off-screen render target and then apply the transformation before presentation.
This patch negates the need for that off-screen render pass.

To be clear, the previous architecture is still fully viable. Embedders still
have full control over layer transformations before composition. This is an
optimization for the hardware overlay planes use-case.

Fixes b/139758641

Fixes https://github.com/flutter/flutter/issues/38870

This patch allows embedders to split the Flutter layer tree into multiple
chunks. These chunks are meant to be composed one on top of another. This gives
embedders a chance to interleave their own contents between these chunks.

The Flutter embedder API already provides hooks for the specification of
textures for the Flutter engine to compose within its own hierarchy (for camera
feeds, video, etc..). However, not all embedders can render the contents of such
sources into textures the Flutter engine can accept. Moreover, this composition
model may have overheads that are non-trivial for certain use cases. In such
cases, the embedder may choose to specify multiple render target for Flutter to
render into instead of just one.

The use of this API allows embedders to perform composition very similar to the
iOS embedder. This composition model is used on that platform for the embedding
of UIKit view such and web view and map views within the Flutter hierarchy.
However, do note that iOS also has threading configurations that are currently
not available to custom embedders.

The embedder API updates in this patch are ABI stable and existing embedders
will continue to work are normal. For embedders that want to enable this
composition mode, the API is designed to make it easy to opt into the same in an
incremental manner.

Rendering of contents into the “root” rendering surface remains unchanged.
However, now the application can push “platform views” via a scene builder.
These platform views need to handled by a FlutterCompositor specified in a new
field at the end of the FlutterProjectArgs struct.

When a new platform view in introduced within the layer tree, the compositor
will ask the embedder to create a new render target for that platform view.
Render targets can currently be OpenGL framebuffers, OpenGL textures or software
buffers. The type of the render target returned by the embedder must be
compatible with the root render surface. That is, if the root render surface is
an OpenGL framebuffer, the render target for each platform view must either be a
texture or a framebuffer in the same OpenGL context. New render target types as
well as root renderers for newer APIs like Metal & Vulkan can and will be added
in the future. The addition of these APIs will be done in an ABI & API stable
manner.

As Flutter renders frames, it gives the embedder a callback with information
about the position of the various platform views in the effective hierarchy.
The embedder is then meant to put the contents of the render targets that it
setup and had previously given to the engine onto the screen (of course
interleaving the contents of the platform views).

Unit-tests have been added that test not only the structure and properties of
layer hierarchy given to the compositor, but also the contents of the texels
rendered by a test compositor using both the OpenGL and software rendering
backends.

Fixes b/132812775
Fixes flutter/flutter#35410

This exposes the `Settings::leak_vm` flag to custom embedders. All embedder
unit-tests now shut down the VM on the shutdown of the last engine in the
process. The mechanics of VM shutdown are already tested in the Shell unit-tests
harness in the DartLifecycleUnittests set of of assertions. This just exposes
that functionality to custom embedders. Since it is part of the public stable
API, I also switched the name of the field to be something less snarky than the
field in private shell settings.

embedder.h is a C API, so has no namespace, and only uses 'Flutter' as a
prefix for most symbol names. This creates potential collisions with
other code; for instance, FlutterEngine is the name of a type in
embedder.h, but also an ObjC class in the iOS Flutter API.

This adds a macro that can be set to prefix symbol names, allowing
clients (notably, the macOS embedding) to adjust the names used by the
embedding API internally without breaking ABI or API compatibility for
the standard engine build.

Currently the macro is only applied to FlutterEngine, since that's the
symbol that is currently at issue, but it can be expanded to other
symbols in the future.

Fixes https://github.com/flutter/flutter/issues/18852

Messages sent to the embedder host may be one-way messages with no response
handler.  If the host calls FlutterEngineSendPlatformMessageResponse on a
one-way message, then just delete the message response handle.

Also update the documentation to indicate that
FlutterEngineSendPlatformMessageResponse must be called for all messages.
Previously the docs implied that some FlutterPlatformMessage objects may
have a null response_handle.  The embedder will now set a response_handle for
every message (even if the sender does not expect a response).

Corects a bnuch of typeos throughout teh engien codebsae. Also makes
a couple minor Commonwealth -> US spelling adjustments for consistency
with the rest of Flutter's codebase.

Made use of `misspell` tool:
https://github.com/client9/misspell

Rather than hard-coding the type of incoming events to mouse, and
synthesizing a primary button press for kDown/kUp, expose device kind
and buttons in the API.

For backwards compatibility, if the type is not set, the old behavior is
used. If an embedder sets the type to mouse explicitly, however, they
must also set correct button information.

For the touch type, the API abstracts away the framework's internal
expectation that a button is set for touch down/move for simplicity.

Fixes flutter/flutter#32854

PR #8498 made these constants extern, but forgot to export them so they
would be public symbols.