[skotty,sksg] Initial matte layer support

TBR=
Change-Id: I5b689f5d7b0d147fa200cf5bffe476077085cb19
Reviewed-on: https://skia-review.googlesource.com/93300
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed