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>
diff --git a/gn/effects.gni b/gn/effects.gni
index da0ce8c..09fac9f 100644
--- a/gn/effects.gni
+++ b/gn/effects.gni
@@ -52,6 +52,7 @@
   "$_src/effects/SkPaintImageFilter.cpp",
   "$_src/effects/SkPictureImageFilter.cpp",
   "$_src/effects/SkRRectsGaussianEdgeMaskFilter.cpp",
+  "$_src/effects/SkShaderMaskFilter.cpp",
   "$_src/effects/SkTableColorFilter.cpp",
   "$_src/effects/SkTableMaskFilter.cpp",
   "$_src/effects/SkTileImageFilter.cpp",