[skottie] Initial mask support

TBR=

Change-Id: Ibf4baeb17f98e1ed359f04deefe2f1c09414540d
Reviewed-on: https://skia-review.googlesource.com/98840
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed