[skottie] Add support for gradient opacity stops

Bug: skia:9600
Change-Id: If95c02279efae21db8a673a3b0d349915b0aaba0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252483
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed