Bundle shared SkImage_Gpu methods.

Also adds an enum to make YUV index more explicit.

Bug: skia:7903
Change-Id: I21ae4cac126275fa7fd257235910fd3be155d293
Reviewed-on: https://skia-review.googlesource.com/c/159160
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
8 files changed