[skif] Combine SkBlendImageFilter and SkArithmeticImageFilter

Change-Id: Ie0e8eae775ce76ee6f27b05d2c97a2b21e807e2b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/699218
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
13 files changed