Moved our code to passing functions by const ref

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

* Moves pointer event sanitizing to engine

* fix comment format

* fix formatting

* addressing comment

* fix format

* fix format

* addressing comment

This reverts commit f456423c.

This is being reverted because it caused flutter/flutter#45098
(images don't load on iOS).

This incorrect assumption led to the introduction of a failure on an external embedder. Also dries up the section that copies the picture to the embedder managed render targets.

Fixes https://github.com/flutter/flutter/issues/43906
Fixes https://b.corp.google.com/issues/143529469

This reverts commit 2dcfaaeb.

This is a duplicate of flutter/engine#13360 with the test switched to use the software backend instead of the GL backend.

After some debugging and testing on another GL embedder I think the issue with the test is some bug having to do with the GL implementation in the test harness specifically. 

Fixes flutter/flutter#38903

This reverts commit 618e6666.

Revert "Always use `IOSGLContextSwitch` to access EAGLContexts to prevent plugins from polluting Flutter's EAGLContext (#13314)" (#13753)

Always use `IOSGLContextSwitch` to access EAGLContexts to prevent plugins from polluting Flutter's EAGLContext (#13314)

The earlier assumption was that the render target would be re-materialized per frame. The render target needs its own picture recorder to be create per frame as well. When render targets are cached in the registry, an existing target will be reused. But submitting the previous frame would have discarded the recorder already. The layer tree paint would then attempt to dererence a null canvas causing a crash at runtime.

Added tests to ensure that this does not happen both with and without a custom compositor specified by the embedder. I am going to rework this code so that the external view embedders thinks of render target access on a per frame basis but that is a larger change. This smaller patchset should unblock broken builds.

Fixes b/144093523

Ensure that the device pixel ratio is taken into account with window metrics in physical pixels. (#13708)

This was originally patched in https://github.com/flutter/engine/pull/13193 but
the unit-tests were written under the mistaken assumption (that has been
documented already) that the Flutter window metrics used logical pixel
coordinates. That mistake has been corrected and additional tests to verify
rendering intent has been added.

Fixes https://github.com/flutter/flutter/issues/43906
Fixes https://b.corp.google.com/issues/143529469

This caused EmbedderTest.VerifyB143464703 to fail after merging into
master.

```
../../flutter/shell/platform/embedder/tests/embedder_unittests.cc:3111: Failure
Value of: ImageMatchesFixture("verifyb143464703.png", renderered_scene)
  Actual: false
Expected: true
[  FAILED  ] EmbedderTest.VerifyB143464703 (2507 ms)
```

This reverts commit 3ad3bc76.

Previously the cache was disabled on whether or not PlatformViews were
globally enabled. Instead track their existence in the view hierarchy
and only disable RasterCache if a PlatformView is actually present.

This used to only be handled correctly for non-root layer backing stores. This
was mostly a side effect of the fact that we used recording canvases instead of
rendering directly into the backing store. We now use recording canvases
consistently.

Fixes b/143464703
Fixes https://github.com/flutter/flutter/issues/43732

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.

Removes a TODO implying that the embedder accessibility unittests were
disabled due to flakiness.

The tests were originally disabled in
https://github.com/flutter/engine/pull/9482.

The issue tracking re-enabling the tests was
https://github.com/flutter/flutter/issues/35218.

This issue was resolved, and the tests re-enabled, in
https://github.com/flutter/engine/pull/9585.

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.

Fixes https://github.com/flutter/flutter/issues/42480

This reverts commit 6c2381da and applies iOS fixes.

This reverts commit e53d10c3.

The contents rendered into the backing stores are already correctly scaled.
The initial implementation assumed this also held true for the metrics obtained
via embedded view parameters.

Fixes b/142699417

Fixes https://github.com/flutter/flutter/issues/42704.

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

* Remove uses of the "blobs" snapshot format in Engine tests.

* Comments

* Rebasing against updated ELF loader API.

Texture unregistration is finished on the GPU thread. The FlutterTexture implementation might not know when it is finished which leads to a race condition. Adding this callback so the FlutterTexture is aware of end of the unregistration process.

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

Unit tests added:
- CacheSkSLWorks
- VisitFilesCanBeCalledTwice
- CanListFilesRecursively

Add a unit-test to verify that root surface transformation affect platform view coordinates. (#12783)

See b/141980393 for details.

In the issue, the embedder (assumed to render Flutter contents of size 800 x 600 [1]) is meant to be displayed on its side. To achieve this, it specifies a root surface transformation that translates the surface by its width (or height when it held in the correct viewing position) and then rotates it counter-clockwise by 90 degrees. This test verifies that the Flutter Engine accounts for those transformations in the custom compositor platform view coodinates.

[1] The actual size is something different. 800x600 is for illustrative purposes.

This reverts commit 974ca210.