[skottie] Apply fully opaque masks as clips

We already have a clip optimization when there is only one opaque mask.

Extend to cover multiple opaque masks, using Merge scene nodes.

TBR=
Change-Id: I24b61f0c0d080b13438c6777e98a8e2fefd09fdd
Reviewed-on: https://skia-review.googlesource.com/140002
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed