Use texture barriers to read directly from the RT

src/gpu/GrXferProcessor.cpp

Index: src/gpu/GrXferProcessor.cpp
diff --git a/src/gpu/GrXferProcessor.cpp b/src/gpu/GrXferProcessor.cpp
index 61cacac62ee8b6d1188cf3d9083de523d2b415b0..defa884bd2d824e11d5813743ff5808e9c9fc1bb 100644
--- a/src/gpu/GrXferProcessor.cpp
+++ b/src/gpu/GrXferProcessor.cpp
@@ -32,6 +32,19 @@ void GrXferProcessor::getGLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBu
     this->onGetGLProcessorKey(caps, b);
 }
 
+bool GrXferProcessor::willNeedXferBarrier(const GrRenderTarget* rt,
+                                          const GrDrawTargetCaps& caps,
+                                          GrXferBarrierType* outBarrierType) const {
+    if (static_cast<const GrSurface*>(rt) == this->getDstCopyTexture()) {
+        // Texture barriers are required when a shader reads and renders to the same texture.
+        SkASSERT(rt);
+        SkASSERT(caps.textureBarrierSupport());
+        *outBarrierType = kTexture_GrXferBarrierType;
+        return true;
+    }
+    return false;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 GrXferProcessor* GrXPFactory::createXferProcessor(const GrProcOptInfo& colorPOI,