Fix some bugs around opaque color types and computeIsOpaque

Spun out from: https://skia-review.googlesource.com/c/skia/+/161423

Bug: skia:
Change-Id: I7f3a7e10faa844271235f3b064576d43bab8e554
Reviewed-on: https://skia-review.googlesource.com/c/161480
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed