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

src/gpu/GrDrawState.h

Index: src/gpu/GrDrawState.h
===================================================================
--- src/gpu/GrDrawState.h	(revision 8451)
+++ src/gpu/GrDrawState.h	(working copy)
@@ -507,6 +507,18 @@
             }
         }
     }
+    
+    /**
+     * 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;
+    }
 
     /// @}