Separate SkArithmeticImageFilter from SkXfermodeImageFilter.

Change-Id: I145eed7276456b546ca5c66bc1a0f0f74a84f138
Reviewed-on: https://skia-review.googlesource.com/6728
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
13 files changed