Don't make dst copies when color stage requires dst but color writes are disabled.

src/gpu/GrDrawState.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrDrawState.h"
 #include "GrPaint.h"
 
@@ skipping 145 matching lines @@
                     return false;
                 }
                 slTypes[attribIndex] = effectSLType;
             }
         }
     }
 
     return true;
 }
 
+bool GrDrawState::willEffectReadDstColor() const {
+    int startStage = this->isColorWriteDisabled() ? this->getFirstCoverageStage() : 0;
+    for (int s = startStage; s < kNumStages; ++s) {
+        if (this->isStageEnabled(s) && (*this->getStage(s).getEffect())->willReadDstColor()) {
+            return true;
+        }
+    }
+    return false;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 bool GrDrawState::srcAlphaWillBeOne() const {
     uint32_t validComponentFlags;
     GrColor color;
     // Check if per-vertex or constant color may have partial alpha
     if (this->hasColorVertexAttribute()) {
         validComponentFlags = 0;
         color = 0; // not strictly necessary but we get false alarms from tools about uninit.
     } else {
@@ skipping 263 matching lines @@
             }
             fRestoreMask |= (1 << s);
             GrEffectStage* stage = drawState->fStages + s;
             stage->saveCoordChange(&fSavedCoordChanges[s]);
             stage->localCoordChange(invVM);
         }
     }
     drawState->viewMatrix()->reset();
     return true;
 }