[graphite] Specialization for un/premul color xforms

Bug: b/388849244
Change-Id: Iaeb95403bab2fef3d3b29b60fb565eea86febf6c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/936864
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
12 files changed