Make FP optimizations helpers use SkAlphaType not GrColorType

Change-Id: I3b7cde6e7f68192af5419fb09c7d59e7a803a4d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255818
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
58 files changed