Move willUseInputColor check to computeInvariantOutput

src/gpu/effects/GrConvexPolyEffect.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gl/builders/GrGLProgramBuilder.h"
 #include "GrConvexPolyEffect.h"
 
@@ skipping 28 matching lines @@
     }
 
     virtual bool onIsEqual(const GrFragmentProcessor& other) const SK_OVERRIDE {
         const AARectEffect& aare = other.cast<AARectEffect>();
         return fRect == aare.fRect;
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
         if (fRect.isEmpty()) {
             // An empty rect will have no coverage anywhere.
-            inout->setToTransparentBlack();
+            inout->mulByKnownAlpha(0);
         } else {
             inout->mulByUnknownAlpha();
         }
     }
 
     SkRect              fRect;
     GrPrimitiveEdgeType fEdgeType;
 
     typedef GrFragmentProcessor INHERITED;
 
@@ skipping 310 matching lines @@
     }
 
     GrFragmentProcessor* fp;
     do {
         GrPrimitiveEdgeType edgeType = static_cast<GrPrimitiveEdgeType>(
                                         random->nextULessThan(kGrProcessorEdgeTypeCnt));
         fp = GrConvexPolyEffect::Create(edgeType, count, edges);
     } while (NULL == fp);
     return fp;
 }