added support for SkSL unpremul function

Change-Id: I970f1ad0dd0859448c874498fe02342f8abc3aa3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242897
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
14 files changed