consolidate read/writePixels in GrSurfaceContext

Change-Id: I118fcd49990597d4dfea92efd3f9d99e52fdbfab
Reviewed-on: https://skia-review.googlesource.com/11481
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed