asyncRescaleAndRead supports unpremul results on GPU

GrSurfaceContext::rescale uses GrSurfaceFillContext instead of
GrSurfaceDrawContext.
Change-Id: I9c2d647d8f221c129ec4485a4ed936202aee6362
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/351923
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
9 files changed