[skottie] Linear Wipe effect

Add SkMaskFilter scene graph nodes and implement AE's linear wipe effect
as a gradient shader mask filter.

Change-Id: I3b2d8677c894d27249cfae7e3ea6b1248b53546b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/221776
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
9 files changed