Use texture barriers to read directly from the RT

include/gpu/GrXferProcessor.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrXferProcessor_DEFINED
 #define GrXferProcessor_DEFINED
 
@@ skipping 246 matching lines @@
     virtual void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                                     InvariantOutput*) const = 0;
 
     /**
      * Determines whether multiplying the computed per-pixel color by the pixel's fractional
      * coverage before the blend will give the correct final destination color. In general it
      * will not as coverage is applied after blending.
      */
     virtual bool canTweakAlphaForCoverage() const = 0;
 
+    /**
+     * Returns whether the Xfer processor will blend coherently with draw calls that have been
+     * submitted previously. This is generally true, however some blending extensions require the
+     * use of a barrier.
+     */
+    virtual bool willBlendCoherently(const GrDrawTargetCaps& caps) const = 0;
+
     bool willNeedDstCopy(const GrDrawTargetCaps& caps, const GrProcOptInfo& colorPOI,
                          const GrProcOptInfo& coveragePOI) const;
 
     bool isEqual(const GrXPFactory& that) const {
         if (this->classID() != that.classID()) {
             return false;
         }
         return this->onIsEqual(that);
     }
 
@@ skipping 44 matching lines @@
     enum {
         kIllegalXPFClassID = 0,
     };
     static int32_t gCurrXPFClassID;
 
     typedef GrProgramElement INHERITED;
 };
 
 #endif