use faster cases in blitAntiH for SkVMBlitter

Use the coverage from the mask to avoid or reduce blitting.

nanobench
desk_chalkboard 51ms -> 38ms.

Change-Id: I58e3ca5f9ef59dbd5b9ceb1a905ea1f3d948fd89
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/461296
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed