Previously, FlutterSurfaceManager was a protocol with two concrete
implementations: FlutterGLSurfaceManager and FlutterMetalSurfaceManager.
Most of the implementation was in a shared superclass,
FlutterIOSurfaceManager, which called into the OpenGL or Metal-specific
subclass when backend-specific operations (such as allocating textures)
was required. It did so via a delegate pattern, wherein the subclasses
both implemented the FlutterIOSurfaceManagerDelegate protocol that
exposed the backend-specific functionality.
Now that only the Metal implementation remains, the delegate code can be
inlined into the calling functions, and the class hierarchy can be
squashed into a single concrete implementation class,
FlutterSurfaceManager, similar to how it was originally implemented in
https://github.com/flutter/engine/pull/21525 before we had two backends.
Issue: https://github.com/flutter/flutter/issues/108304
Issue: https://github.com/flutter/flutter/issues/114445
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