start adding premul tests

Not sure why I could only check the red channels in the PremulLinear
cases.  I guess the green and blue channels were right on the edge?

Change-Id: Ib83f94132c0121bc488c7b6162910c744c412e97
Reviewed-on: https://skia-review.googlesource.com/112880
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed