Re-re-land "added GrSkSLFP and converted DitherEffect to use it"

This reverts commit 6c48e4d11ce80fa5cfef56e13b2d5847fe94a7cc.

Bug: skia:
Change-Id: I7ee78990fc30eec545d1856e59eb6e0573089426
Reviewed-on: https://skia-review.googlesource.com/144348
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index 5cf376f..fa04c8e 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -18,6 +18,7 @@
   "$_src/sksl/SkSLLexer.cpp",
   "$_src/sksl/SkSLMetalCodeGenerator.cpp",
   "$_src/sksl/SkSLParser.cpp",
+  "$_src/sksl/SkSLPipelineStageCodeGenerator.cpp",
   "$_src/sksl/SkSLSPIRVCodeGenerator.cpp",
   "$_src/sksl/SkSLString.cpp",
   "$_src/sksl/SkSLUtil.cpp",
@@ -36,7 +37,6 @@
   "$_src/gpu/effects/GrCircleEffect.fp",
   "$_src/gpu/effects/GrConfigConversionEffect.fp",
   "$_src/gpu/effects/GrConstColorProcessor.fp",
-  "$_src/gpu/effects/GrDitherEffect.fp",
   "$_src/gpu/effects/GrEllipseEffect.fp",
   "$_src/gpu/effects/GrLumaColorFilterEffect.fp",
   "$_src/gpu/effects/GrMagnifierEffect.fp",