Add more specialized fragment builders

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index c1f9141a716c80a6d9f01127ba2afc3cb8069dd9..79520602b719e69f169f7511415861e3d390f12f 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -164,7 +164,7 @@ void GrGLAlphaThresholdEffect::emitCode(EmitArgs& args) {
                                                     kFloat_GrSLType, kDefault_GrSLPrecision,
                                                     "outer_threshold");
 
-    GrGLSLFragmentBuilder* fragBuilder = args.fFragBuilder;
+    GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
     SkString coords2D = fragBuilder->ensureFSCoords2D(args.fCoords, 0);
     SkString maskCoords2D = fragBuilder->ensureFSCoords2D(args.fCoords, 1);