[graphite] Implement SkSurface::makeImageSnapshot as a copy

Bug: skia:b/238755730
Change-Id: I35ce99dee91f080e732cab3e0e5b83ff6bc277b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/585151
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
13 files changed