Use texture barriers to read directly from the RT

src/gpu/GrDrawTarget.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDrawTarget_DEFINED
 #define GrDrawTarget_DEFINED
 
@@ skipping 748 matching lines @@
                                   const SkIRect& srcRect,
                                   const SkIPoint& dstPoint) = 0;
     /**
      * This will be called before allocating a texture to be a dst for onCopySurface. Only the
      * dstDesc's config, flags, and origin need be set by the function. If the subclass cannot
      * create a surface that would succeed its implementation of onCopySurface, it should return
      * false. The base class will fall back to creating a render target to draw into using the src.
      */
     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,
                             void** vertices);
     bool reserveIndexSpace(int indexCount, void** indices);
 
     // called by drawIndexed and drawNonIndexed. Use a negative indexCount to
     // indicate non-indexed drawing.
     bool checkDraw(const GrPipelineBuilder&,
                    const GrGeometryProcessor*,
@@ skipping 74 matching lines @@
     virtual bool setupClip(GrPipelineBuilder*,
                            GrPipelineBuilder::AutoRestoreFragmentProcessors*,
                            GrPipelineBuilder::AutoRestoreStencil*,
                            GrScissorState* scissorState,
                            const SkRect* devBounds) override;
 
     typedef GrDrawTarget INHERITED;
 };
 
 #endif