ComposeShader GPU implementations

Bug: skia:
Change-Id: I007b46ead1900a8f5cce82dec33b6d662fc63d8e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206918
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
10 files changed