Use texture barriers to read directly from the RT

src/gpu/GrGpu.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 GrGpu_DEFINED
 #define GrGpu_DEFINED
 
@@ skipping 286 matching lines @@
 
     // Called to perform a surface to surface copy. Fallbacks to issuing a draw from the src to dst
     // take place at the GrDrawTarget level and this function implement faster copy paths. The rect
     // and point are pre-clipped. The src rect and implied dst rect are guaranteed to be within the
     // src/dst bounds and non-empty.
     virtual bool copySurface(GrSurface* dst,
                              GrSurface* src,
                              const SkIRect& srcRect,
                              const SkIPoint& dstPoint) = 0;
 
+    // Inserts a boundary between render passes that touch the same pixels. Some blending extensions
+    // require this in order to guarantee coherent results.
+    virtual void blendBarrier() = 0;

[Mark Kilgard, 2015/04/02 23:02:38]

seems like you could just provide a no-op implementation {} rather than a base
virtual method

having a no-op default blendBarrier makes sense to me

long term, I don't expect hardware to need the barrier to do anything

+
     struct DrawArgs {
         typedef GrDrawTarget::DrawInfo DrawInfo;
         DrawArgs(const GrPrimitiveProcessor* primProc,
                  const GrPipeline* pipeline,
                  const GrProgramDesc* desc,
                  const GrBatchTracker* batchTracker)
             : fPrimitiveProcessor(primProc)
             , fPipeline(pipeline)
             , fDesc(desc)
             , fBatchTracker(batchTracker) {
@@ skipping 202 matching lines @@
     int                                                                 fGpuTraceMarkerCount;
     GrTraceMarkerSet                                                    fActiveTraceMarkers;
     GrTraceMarkerSet                                                    fStoredTraceMarkers;
     // The context owns us, not vice-versa, so this ptr is not ref'ed by Gpu.
     GrContext*                                                          fContext;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif