Arithmetic xfermode stage.

565 is identical, and as usual the old sRGB path was broken.  :)
It was unimplemented, so the default missing Sk4f implementation was mimicking Src.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2175133002

Review-Url: https://codereview.chromium.org/2175133002
1 file changed