Merge common code for SkImage_Gpu and SkImage_GpuYUVA.

Bug: skia:7903
Change-Id: I29957babe65e9e1c505166d01490233fe542a559
Reviewed-on: https://skia-review.googlesource.com/c/169824
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed