[skotty] Initial opacity support

TBR=

Change-Id: I62581d3a7a83af5ccf373f0f4edf66a2d7f06f07
Reviewed-on: https://skia-review.googlesource.com/92223
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed