Remove GrArithmeticProcessor, use GrRuntimeFPBuilder instead

This also converts "enforcePMColor" to a uniform (rather than baking it
in). It's easier to write this way, and I can't imagine it made that
much difference to performance.

Change-Id: Idd202678740077412a2fb4e5122ebb9118f1d7bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/410096
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
8 files changed