Trim the copy of the dst made for dst-reading effects using the clip and dev-bounds of draw.

src/gpu/GrDrawState.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDrawState_DEFINED
 #define GrDrawState_DEFINED
 
@@ skipping 489 matching lines @@
      * correct local coordinates. oldToNew gives the transformation from the old coord system in
      * which the geometry was specified to the new coordinate system from which it will be rendered.
      */
     void localCoordChange(const SkMatrix& oldToNew) {
         for (int i = 0; i < kNumStages; ++i) {
             if (this->isStageEnabled(i)) {
                 fStages[i].localCoordChange(oldToNew);
             }
         }
     }
+    
+    /**
+     * Checks whether any of the effects will read the dst pixel color.
+     */
+    bool willEffectReadDst() const {
+        for (int s = 0; s < kNumStages; ++s) {
+            if (this->isStageEnabled(s) && (*this->getStage(s).getEffect())->willReadDst()) {
+                return true;
+            }
+        }
+        return false;
+    }
 
     /// @}
 
     ///////////////////////////////////////////////////////////////////////////
     /// @name Coverage / Color Stages
     ////
 
     /**
      * A common pattern is to compute a color with the initial stages and then
      * modulate that color by a coverage value in later stage(s) (AA, mask-
@@ skipping 705 matching lines @@
     int                                    fAttribIndices[kAttribIndexCount];
     GrVertexAttribArray<kVertexAttribCnt>  fVertexAttribs;
     GrEffectStage                          fStages[kNumStages];
 
     typedef GrRefCnt INHERITED;
 };
 
 GR_MAKE_BITFIELD_OPS(GrDrawState::BlendOptFlags);
 
 #endif