Reland "[graphite] Add Caps::channelMask()"

Bug: b/238756380
Change-Id: Ia114677edc045a9f76ad8c769b7200a48977be1a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/653896
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
10 files changed