[graphite] Implement fullscreen clear as LoadOp

Bug: skia:12466
Change-Id: Ie8bbacd22375b4cb4a5fe7e39055cdba3a3d0896
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/469519
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
7 files changed