[graphite] Short circuit key generation for simple paints

If there is no shader and no primitive color blending then the input to the color filter stage is just the premultiplied paint color.

Bug: b/251493570
Change-Id: I0f3aa92787dde905122cf808e1de12c13f13acc9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/761278
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
2 files changed