Remove checks that insist on kOpaque for some color types

For destinations, Premul vs. Unpremul is actually meaningful
for these color types - it determines whether or not we divide
through by alpha after blending. Opaqueness ought to just be
a performance hint, and only for sources.

Bug: skia:
Change-Id: I801a64316ec9c92642982cd2096f8bec9a7855f3
Reviewed-on: https://skia-review.googlesource.com/c/161423
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed