Merge "Revert float xfermodes back to Sk4f (from Sk8f)." into master-skia

https://skia.googlesource.com/skia/+/29d60e5ab594b39d1f533bff090877b1bb821e06

Change-Id: I6370bef5cfc46c8fc9863f71d0da9b282398b2e8