[graphite] Add paint's contribution when using alpha-only textures

Change-Id: Iffc9e46ab50856baafc212fce7eeec066e214c51
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/647677
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed