Convert the dither runtime effect into a regular .fp effect.

This was previously implemented as a runtime effect in order to dogfood
runtime effects when they were new. However, runtime effects do not
support passing a color to a FP via `sample(fp, inputColor)`, which
is needed to properly implement SkPaintToGrPaint.

Change-Id: Ic826860ab06038f13633be677d79a9138d41db85
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301547
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index aa80bd7..6c0454a 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -88,6 +88,7 @@
   "$_src/gpu/effects/GrConfigConversionEffect.fp",
   "$_src/gpu/effects/GrConstColorProcessor.fp",
   "$_src/gpu/effects/GrDeviceSpaceEffect.fp",
+  "$_src/gpu/effects/GrDitherEffect.fp",
   "$_src/gpu/effects/GrEllipseEffect.fp",
   "$_src/gpu/effects/GrHSLToRGBFilterEffect.fp",
   "$_src/gpu/effects/GrLumaColorFilterEffect.fp",