add gm to test alpha in drawPatch

Bug: skia:
Change-Id: I79d0b0707b09a3eedeeaee940d6dafbbbdbe2fe2
Reviewed-on: https://skia-review.googlesource.com/17707
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed