Move willUseInputColor check to computeInvariantOutput

src/effects/SkArithmeticMode.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 "SkArithmeticMode.h"
 #include "SkColorPriv.h"
 #include "SkReadBuffer.h"
@@ skipping 328 matching lines @@
            fEnforcePMColor == s.fEnforcePMColor &&
            backgroundTexture() == s.backgroundTexture();
 }
 
 const GrBackendFragmentProcessorFactory& GrArithmeticEffect::getFactory() const {
     return GrTBackendFragmentProcessorFactory<GrArithmeticEffect>::getInstance();
 }
 
 void GrArithmeticEffect::onComputeInvariantOutput(InvariantOutput* inout) const {
     // TODO: optimize this
-    inout->setToUnknown();
+    inout->setToUnknown(InvariantOutput::kWill_ReadInput);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 GrGLArithmeticEffect::GrGLArithmeticEffect(const GrBackendProcessorFactory& factory,
                                            const GrProcessor&)
    : INHERITED(factory),
      fEnforcePMColor(true) {
 }
 
@@ skipping 92 matching lines @@
                                          background);
     }
     return true;
 }
 
 #endif
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkArithmeticMode)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkArithmeticMode_scalar)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END