Use GrSurfaceContext::read/write-Pixels more

Change-Id: I04bcaf91aa7a43e0563e332c1fe2836d762a04d4
Reviewed-on: https://skia-review.googlesource.com/7520
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed