[graphite] Add support for synchronous surface readpixels

Bug: skia:12466
Change-Id: Ia0da94fc300ab678712e9bb79a011694fea8595e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/464677
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
8 files changed