[graphite] Add hooks for asyncRescaleAndReadPixels

Bug: b/238756873
Change-Id: Ib977548b7d27670068fc0481bcf0830518d36a53
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/585960
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed