Add more specialized fragment builders

src/gpu/effects/GrDitherEffect.cpp

Index: src/gpu/effects/GrDitherEffect.cpp
diff --git a/src/gpu/effects/GrDitherEffect.cpp b/src/gpu/effects/GrDitherEffect.cpp
index 43fe93769aaca848d3fd3c5df4e6a3e3e7e469a9..39d4063a5892255731359d18443244edb17655c8 100644
--- a/src/gpu/effects/GrDitherEffect.cpp
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -67,7 +67,7 @@ private:
 };
 
 void GLDitherEffect::emitCode(EmitArgs& args) {
-    GrGLSLFragmentBuilder* fragBuilder = args.fFragBuilder;
+    GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
     // Generate a random number based on the fragment position. For this
     // random number generator, we use the "GLSL rand" function
     // that seems to be floating around on the internet. It works under