Separate out ANGLE renderer to prepare for DX12

[ds84182]

Part of flutter/flutter#108486

This introduces the concept of a "WindowSurface" which manages everything rendering-related for a PlatformWindow. The WindowSurface would also provide a compositor implementation for platform views.

AngleSurfaceManager should be moved into WindowSurface, but first it'll require some changes to the texture registrar.

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