add interp-in-premul case to GM

Change-Id: I9811c76c3db38d5ab49ba0312e433aa46744df05
Reviewed-on: https://skia-review.googlesource.com/155781
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed