spin off unpremul and opaque tests

  - opaque and unpremul are never the same...
    there's no reason to ever premul opaque sources.

  - under optimization, they won't always be a deterministic
    tweak the the premul steps (though right now they still are).

Change-Id: I5669b3dba83774326c07d5a8f16b1d2ce2b22aae
Reviewed-on: https://skia-review.googlesource.com/130061
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@chromium.org>
2 files changed