Potentially optimize some GrGLEffects for known input color values

src/effects/SkBitmapAlphaThresholdShader.cpp

Index: src/effects/SkBitmapAlphaThresholdShader.cpp
diff --git a/src/effects/SkBitmapAlphaThresholdShader.cpp b/src/effects/SkBitmapAlphaThresholdShader.cpp
index 4337afdbf4f0ded712ab82d82d3c08c2c3fac118..93cac1051e89d8371b56fa2fcfb139105d815f66 100644
--- a/src/effects/SkBitmapAlphaThresholdShader.cpp
+++ b/src/effects/SkBitmapAlphaThresholdShader.cpp
@@ -116,7 +116,7 @@ public:
                               const GrDrawEffect& drawEffect,
                               EffectKey key,
                               const char* outputColor,
-                              const char* inputColor,
+                              const GrGLSLExpr4& inputColor,
                               const TransformedCoordsArray& coords,
                               const TextureSamplerArray& samplers) SK_OVERRIDE {
             // put bitmap color in "color"
@@ -150,7 +150,7 @@ public:
                                     "\t\t}\n");
 
             builder->fsCodeAppendf("color = %s = %s;\n", outputColor,
-                                   (GrGLSLExpr4(inputColor) * GrGLSLExpr4("color")).c_str());
+                                   (inputColor * GrGLSLExpr4("color")).c_str());
         }
 
         virtual void setData(const GrGLUniformManager& uman, const GrDrawEffect& e) SK_OVERRIDE {