split out blendmodes for skvm

Idea is to make shared functions around blendmodes, so we can use them from the
blitter-builder, and from effects like ModeColorFilter.

This CL just tries the refactor. After this, will be easy/fun to flesh out (some)
more of the other modes.

Change-Id: Ia55c60099065c16efdc322a5cab425e222540d6a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252658
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed