compose and combine maskfilters

Bug: skia:
Change-Id: Id470124021d00ee25cf4ae392a4ee345ced84110
Reviewed-on: https://skia-review.googlesource.com/97760
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed