[skottie] Mask feather support

Wrap mask nodes with a sksg::BlurImageFilter to implement the feather
(blur) effect.

Requires assets exported with https://github.com/bodymovin/bodymovin-extension/pull/13.

Change-Id: Ia7809a73582f6ed4e55c88a26b5fe36a52cbb749
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211340
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed