This reverts commit 2e10a974.

This reverts commit 6a2df7f1.

This reverts commit 0d71d27aa7d9058841bbba104021927c7f17b7ce.

This reverts commit 5bf65335.

This patch applies the following changes:

* Rename a stray .cc file to .mm (connection_collection.cc).
* Migrate Objective-C #includes to #import as per the style guide.
* #include/#import order updated to reflect the style guide order:
  associated header, system/standard library headers, library headers,
  platform-specific includes.
* Include cstring where we're using strlen.
* Add a missing copyright header in SemanticsObjectTest.mm.

Bugs: https://github.com/flutter/flutter/issues/60025

This is part of a larger effort to expose the difference between GrDirectContext,
which runs on the GPU thread and can directly perform operations like uploading
textures, and GrRecordingContext, which can only queue up work to be delivered
to the GrDirectContext later.

There is no ability to compile shaders online and cache those binaries when
using the Metal backend. SkSL caching must always be used.

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