Potentially optimize some GrGLEffects for known input color values

src/effects/SkMagnifierImageFilter.cpp

Index: src/effects/SkMagnifierImageFilter.cpp
diff --git a/src/effects/SkMagnifierImageFilter.cpp b/src/effects/SkMagnifierImageFilter.cpp
index e3d6efd4372e5d74a945bc61a6ef36341c6805b3..8fd6170b97225939578ded75c73dc22a5a23f7ae 100644
--- a/src/effects/SkMagnifierImageFilter.cpp
+++ b/src/effects/SkMagnifierImageFilter.cpp
@@ -97,7 +97,7 @@ public:
                           const GrDrawEffect&,
                           EffectKey,
                           const char* outputColor,
-                          const char* inputColor,
+                          const GrGLSLExpr4& inputColor,
                           const TransformedCoordsArray&,
                           const TextureSamplerArray&) SK_OVERRIDE;
 
@@ -119,7 +119,7 @@ void GrGLMagnifierEffect::emitCode(GrGLShaderBuilder* builder,
                                    const GrDrawEffect&,
                                    EffectKey key,
                                    const char* outputColor,
-                                   const char* inputColor,
+                                   const GrGLSLExpr4& inputColor,
                                    const TransformedCoordsArray& coords,
                                    const TextureSamplerArray& samplers) {
     SkString coords2D = builder->ensureFSCoords2D(coords, 0);