Potentially optimize some GrGLEffects for known input color values

src/effects/SkDisplacementMapEffect.cpp

Index: src/effects/SkDisplacementMapEffect.cpp
diff --git a/src/effects/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index 9022b4f69ca561b586b322cb4c6edd2ea66c52b7..ed3a945e24fb181ce805cbc083f0b1cfad912160 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -208,7 +208,7 @@ public:
                           const GrDrawEffect&,
                           EffectKey,
                           const char* outputColor,
-                          const char* inputColor,
+                          const GrGLSLExpr4& inputColor,
                           const TransformedCoordsArray&,
                           const TextureSamplerArray&) SK_OVERRIDE;
 
@@ -407,7 +407,7 @@ void GrGLDisplacementMapEffect::emitCode(GrGLShaderBuilder* builder,
                                          const GrDrawEffect&,
                                          EffectKey key,
                                          const char* outputColor,
-                                         const char* inputColor,
+                                         const GrGLSLExpr4& inputColor,
                                          const TransformedCoordsArray& coords,
                                          const TextureSamplerArray& samplers) {
     sk_ignore_unused_variable(inputColor);