add unpremul gm

Change-Id: I3126fb8b055b58e45f1bd0d913413b4d4d38f032
Reviewed-on: https://skia-review.googlesource.com/c/166740
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed