[skottie] Add support for inverse mask paths

TBR=
Change-Id: I442033b2e82777c90ee497d8a5b2310af1d2e631
Reviewed-on: https://skia-review.googlesource.com/123840
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed