converted Premul / Unpremul to SkSL

Bug: skia:
Change-Id: I4944badbb530e17b9ff7cca389f943e0b5982e01
Reviewed-on: https://skia-review.googlesource.com/72983
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
10 files changed