Add Arithmetic mode xp.

src/gpu/GrOptDrawState.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 "GrOptDrawState.h"
 
 #include "GrDrawState.h"
@@ skipping 137 matching lines @@
     if (flags & GrXferProcessor::kIgnoreCoverage_OptFlag) {
         *firstCoverageStageIdx = ds.numCoverageStages();
     } else {
         if (coveragePOI.readsDst()) {
             fDescInfo.fReadsDst = true;
         }
         if (coveragePOI.readsFragPosition()) {
             fDescInfo.fReadsFragPosition = true;
         }
     }
+
+    if (fXferProcessor->willReadDstColor()) {
+        fDescInfo.fReadsDst = true;
+    }
 }
 
 void GrOptDrawState::finalize(GrGpu* gpu) {
     gpu->buildProgramDesc(*this, fDescInfo, fDrawType, &fDesc);
     fFinalized = true;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 
 bool GrOptDrawState::combineIfPossible(const GrOptDrawState& that) {
@@ skipping 30 matching lines @@
         if (this->getFragmentStage(i) != that.getFragmentStage(i)) {
             return false;
         }
     }
 
     // Now update the GrPrimitiveProcessor's batch tracker
     fPrimitiveProcessor->makeEqual(&fBatchTracker, that.getBatchTracker());
     return true;
 }