[skottie] Apply opaque masks as clips

When the mask stack contains exactly one opaque mask path, we can apply as
a clip.

TBR=
Change-Id: Iadff7534bfa4925557bfbddd59529113f4958d0d
Reviewed-on: https://skia-review.googlesource.com/124000
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed