add shadermaskfilter

next steps:
- gpu impl (will look a lot like shader's asFragProcessor
- special-case rect-input (?)
- explore stages w/ mtklein

Bug: skia:7500
Change-Id: I71089e421d32443a3ddded6967b3e5bc67ed43f2
Reviewed-on: https://skia-review.googlesource.com/95104
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
6 files changed