Groundwork for nonlinear blending mode in Ganesh

Changing a bit of plumbing so we have what we need for one-off color
conversion, and fixing the logic in some of our surface validation.

Plenty more to come, want to submit this stuff in manageable chunks.

BUG=skia:6242

Change-Id: Iac9954c7cc490cc99663db4b9cdf5d6388fc39c7
Reviewed-on: https://skia-review.googlesource.com/9389
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed