Add gm to compare premuls in dst vs. src color space

BUG=skia:

Change-Id: I36755f4bbc405a8af12990573e1f554df012b30a
Reviewed-on: https://skia-review.googlesource.com/9402
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed