Use texture barriers to read directly from the RT

src/gpu/gl/GrGLGpu.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLGpu_DEFINED
 #define GrGLGpu_DEFINED
 
@@ skipping 87 matching lines @@
     bool copySurface(GrSurface* dst,
                      GrSurface* src,
                      const SkIRect& srcRect,
                      const SkIPoint& dstPoint) override;
 
     bool canCopySurface(const GrSurface* dst,
                         const GrSurface* src,
                         const SkIRect& srcRect,
                         const SkIPoint& dstPoint) override;
 
+    void xferBarrier(GrXferBarrierType) override;
+
     void buildProgramDesc(GrProgramDesc*,
                           const GrPrimitiveProcessor&,
                           const GrPipeline&,
                           const GrBatchTracker&) const override;
 
 private:
     // GrGpu overrides
     void onResetContext(uint32_t resetBits) override;
 
     GrTexture* onCreateTexture(const GrSurfaceDesc& desc, GrGpuResource::LifeCycle lifeCycle,
@@ skipping 347 matching lines @@
 
     // we record what stencil format worked last time to hopefully exit early
     // from our loop that tries stencil formats and calls check fb status.
     int fLastSuccessfulStencilFmtIdx;
 
     typedef GrGpu INHERITED;
     friend class GrGLPathRendering; // For accessing setTextureUnit.
 };
 
 #endif