[graphite] Add Caps::channelMask()

Bug: b/238756380
Change-Id: I855a424a1f10a14f521920aabedcaa24404a81ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/650956
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
10 files changed