Use texture barriers to read directly from the RT

src/gpu/GrDrawTarget.h

Index: src/gpu/GrDrawTarget.h
diff --git a/src/gpu/GrDrawTarget.h b/src/gpu/GrDrawTarget.h
index 8a12a31ea9f1d884ac593a5b642d4ba3370d5508..2dabf266508be7f3b41ac1d8a7434abe0ab7a015 100644
--- a/src/gpu/GrDrawTarget.h
+++ b/src/gpu/GrDrawTarget.h
@@ -766,6 +766,13 @@ private:
      */
     virtual bool onInitCopySurfaceDstDesc(const GrSurface* src, GrSurfaceDesc* dstDesc) = 0;
 
+    /**
+     * Inserts a boundary between render passes that touch the same pixels. This prevents
+     * Gr from batching together draw calls for which overlap is illegal, and some blending
+     * extensions require the use of a barrier in order to guarantee coherent results.
+     */
+    virtual void onBlendBarrier() = 0;
+
     // helpers for reserving vertex and index space.
     bool reserveVertexSpace(size_t vertexSize,
                             int vertexCount,