handle non-a8 masks in coverage blitter

Bug: skia:7885
Change-Id: I3ae2d3c7a762d3d718b9a6d40d8c1d696b6aaefc
Reviewed-on: https://skia-review.googlesource.com/124923
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed