Create helper functions to use in computeInvariantOutput calls

src/effects/SkLumaColorFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkLumaColorFilter.h"
 
 #include "SkColorPriv.h"
@@ skipping 96 matching lines @@
     private:
         typedef GrGLFragmentProcessor INHERITED;
     };
 
 private:
     virtual bool onIsEqual(const GrProcessor&) const SK_OVERRIDE {
         return true;
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        // The output is always black.
-        inout->fColor = GrColorPackRGBA(0, 0, 0, GrColorUnpackA(inout->fColor));
-        inout->fValidFlags = kRGB_GrColorComponentFlags;
-        inout->fIsSingleComponent = false;
+        // The output is always black. The alpha value for the color passed in is arbitrary.
+        inout->setToOther(kRGB_GrColorComponentFlags, GrColorPackRGBA(0, 0, 0, 0));
     }
 };
 
 GrFragmentProcessor* SkLumaColorFilter::asFragmentProcessor(GrContext*) const {
     return LumaColorFilterEffect::Create();
 }
 #endif