Split off BlendFormula to be shared with Graphite

Change-Id: Ib9b97eb05a2f40f9de9f43d44afc1480dfdfab83
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/688098
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
6 files changed