Add SkColor4f-accepting SkColorFilters::Blend()

Bug: skia:13637
Change-Id: I56fff2859cee120f679a93a2d59925cba8580962
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/566445
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
6 files changed