The way transactions were added changed in
https://github.com/flutter/engine/commit/68fd8334889610af08cbfc2828f91cf14faf3f1d.
This broke rendering using both Metal and OpenGL when no implicit transaction
was present on the transaction stack. The failure models differ based on Metal
vs. OpenGL and iOS/device versions. On older versions of iOS, rendering would
consume memory till exhaustion. On newer iOS versions, rendering would be stuck
(till a timeout). This patch brings transaction management back in line with as
it was earlier and also makes the Metal backend resilient to transactions being
present on the transaction stack at all. Since this is still quite brittle,
transaction management must be moved to IOSSurface as a followup.

Fixes https://github.com/flutter/flutter/issues/55784.

All Metal layers have their presentsWithTransaction property set to true.
However, when an external view embedder is not present, there is no mechanism to
ensure that the command buffer commit is within transaction scope. This works in
most cases as there there is usually an implicit (possibly nested) transaction
in place during rendering. However, when there isn’t, rendering will look paused
at an incorrect size. This code now works similar to OpenGL but will be
refactored for ease of understanding and consistency between the various
backends.

This moves the Metal `GrContext` creation utilities from `GPUSurfaceMetal` into
a separate `IOSContext` object subclass. An analogue of this object was used in
the GL regime for the management of onscreen and offscreen contexts that were
not tied to the lifecycle of the `GPUSurface`. This pattern has now been
generalized for use with all backends that need a resource context
(`IOSContextGL` and `IOContextMetal`).

The platform views controller management in the `ExternalViewEmbedder` interface
implementation was repeated three times for [Metal][metal], [OpenGL](opengl) and
[Software](software) rendering. This repetition has been removed and a single
implementation present in the base `IOSSurface` and used on all platforms.
Addition of new client rendering APIs should not affect how the engine renders
into the platform view interleaving levels.

All rendering API selection logic has been moved into a single set of utilities
in `rendering_api_selection.h`. This enables the removal of a lot of code blocks
guarded by `FLUTTER_SHELL_ENABLE_METAL`. The remaining uses of this will be
removed when unified builds are enabled.

The Metal backend now also adds traces similar to the GL backend.

The `IOGLContext` has been renamed to `IOContextGL` to be more in line with the
convention used in this library.

Fixes https://github.com/flutter/flutter/issues/41827
Adds https://github.com/flutter/flutter/issues/52150

[metal]: https://github.com/flutter/engine/blob/1194ba2b218706a201c5d2c5325b55a5932546c5/shell/platform/darwin/ios/ios_surface_metal.mm#L55
[opengl]: https://github.com/flutter/engine/blob/1194ba2b218706a201c5d2c5325b55a5932546c5/shell/platform/darwin/ios/ios_surface_gl.mm#L95
[software]: https://github.com/flutter/engine/blob/1194ba2b218706a201c5d2c5325b55a5932546c5/shell/platform/darwin/ios/ios_surface_software.mm#L146

Currently helps primarily on iOS when no BackdropFilter is present by lowering energy usage

* Revert "Add flow test fixtures and tests (#13986)"

This reverts commit 620f5281.

* Revert "Dynamically determine whether to use offscreen surface based on need (#13976)"

This reverts commit a86ef946.

Only use an offscreen surface on iOS if a layer which reads back from the destination surface, such as BackdropFilter, is present.

This reverts commit f456423c.

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

This reverts commit 2dcfaaeb.

By default, the CAMetalLayer backing store is a framebuffer attachment that is
only optimized for display. However, effects such as backdrop filters require
renderbuffer readback. Making this calls will result in exceptions. Disable the
write only optimization on such backing store.

Fixes https://github.com/flutter/flutter/issues/43555

Uses the new `SkSurface::MakeFromCAMetalLayer` Skia API.

Also makes it so that folks don't have to patch the buildroot manually when
building for Metal.