Change GPU read/write pixels API to support color space conversion

GrContext still doesn't convert, but it has the source and destination
color spaces, and call sites are supplying appropriate values where it
makes sense.

BUG=skia:

Change-Id: Ia88733125b8090776cfc9b0dc8030cce365b0b8b
Reviewed-on: https://skia-review.googlesource.com/6400
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed