Use texture barriers to read directly from the RT

include/gpu/GrXferProcessor.h

Index: include/gpu/GrXferProcessor.h
diff --git a/include/gpu/GrXferProcessor.h b/include/gpu/GrXferProcessor.h
index 471271c6158f4a2fe657beab06295b4a9ae43236..abea94b06266e3cd712828f34ccac86f6bb48206 100644
--- a/include/gpu/GrXferProcessor.h
+++ b/include/gpu/GrXferProcessor.h
@@ -264,6 +264,13 @@ public:
      */
     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;