Use texture barriers to read directly from the RT

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index 5784781996b815ab8c0e1e12ace465e563f44232..b2dbec65fb1566b5a74083c8e32865cc0400f1ae 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -304,6 +304,9 @@ public:
                              const SkIRect& srcRect,
                              const SkIPoint& dstPoint) = 0;
 
+    // Called before certain draws in order to guarantee coherent results from dst reads.
+    virtual void xferBarrier(GrXferBarrierType) = 0;
+
     struct DrawArgs {
         typedef GrDrawTarget::DrawInfo DrawInfo;
         DrawArgs(const GrPrimitiveProcessor* primProc,