Now that OpenGL support in Flutter on macOS has been removed, the only FlutterCompositor implementation is the Metal-based compositor. This patch merges it into the FlutterCompositor base class since there's no longer any reason to keep the interface separate from its implementation. This patch will be followed by similar patches for the renderer, surface manager, and external textures. Issue: https://github.com/flutter/flutter/issues/108304 Issue: https://github.com/flutter/flutter/issues/114445