split apart uniformPremul()

It's doing two or three separate things, and I think it's clearer to do
the color conversions and premul up front, leaving just uniformColor().

Change-Id: I65a2f5eac86129924bf30f00a879ae01a1e737a5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319764
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed