skottie_ios_app: Add OpenGL.

  - Consolidate Skottie logic into SkottieViewController class that no longer
    knows about which backend we use.

  - Abstract out SkiaViewController interdace which SkottieViewController
    implements.

  - Create three classes SkiaGLView, SkiaUIView, and SkiaMtkView, which all
    accept SkiaViewController objects but override GLKView, UIView, and
    MTKView.

  - SkAnimationDraw and SkTimeKeeper now SkiaViewController
    implementation details, no longer shared in headers.

Cq-Include-Trybots: skia/skia.primary:Build-Mac-Clang-arm64-Debug-iOS_Metal
Cq-Include-Trybots: skia/skia.primary:Build-Mac-Clang-arm64-Debug-iOS
Change-Id: I96ff2911d63da7d5327c81f91996b2a1b12c4419
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261178
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
23 files changed