Clamp GrSkSLFP output to valid premul

Added this as an option to GrSaturateProcessor (also renamed it to
be more generic and end with FragmentProcessor).

Added a tweak to the unit test to check the new behavior.
(Raster was already doing the clamp).

Change-Id: Ic49fa5cd72b6c63430fb773baf8121546bf2b80d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265580
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed