Plumb through need for unpremul/premul

Update unittest to build test image with legal premul pixels.

Bug: skia:
Change-Id: Iebd1d2f81cac77f8913bd79f6ac25983ed710641
Reviewed-on: https://skia-review.googlesource.com/21735
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed