Express (GLSL expression, possibly known value) pairs as a class

src/effects/SkBitmapAlphaThresholdShader.cpp

Index: src/effects/SkBitmapAlphaThresholdShader.cpp
diff --git a/src/effects/SkBitmapAlphaThresholdShader.cpp b/src/effects/SkBitmapAlphaThresholdShader.cpp
index c8db3a56cad6462ed70a019cbc74d124fe976784..69e22c960824d66070e911284c12d85366416c89 100644
--- a/src/effects/SkBitmapAlphaThresholdShader.cpp
+++ b/src/effects/SkBitmapAlphaThresholdShader.cpp
@@ -149,12 +149,8 @@ public:
                                     "\t\t\tcolor.a = thresh;\n"
                                     "\t\t}\n");
 
-            builder->fsCodeAppend("color = ");
-            SkString outStr;
-            outStr.appendf("\t\t%s = ", outputColor);
-            GrGLSLModulatef<4>(&outStr, inputColor, "color");
-            outStr.append(";\n");
-            builder->fsCodeAppend(outStr.c_str());
+            builder->fsCodeAppendf("color = %s = %s;\n", outputColor,
+                                   (GrGLSLExpr<4>(inputColor) * GrGLSLExpr<4>("color")).c_str());
         }
 
         virtual void setData(const GrGLUniformManager& uman, const GrDrawEffect& e) SK_OVERRIDE {