use mixer to combine two filters

Change-Id: Idb221248606dc683d17f0934b4e3152ff0d6c2d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204360
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
6 files changed