Use texture barriers to read directly from the RT

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 566c8c5b8bbb7f8506838cf992bfe615622904c6..d37f58b86fcc6b996e01caadd8e937972d2c0459 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -2768,6 +2768,13 @@ bool GrGLGpu::canCopySurface(const GrSurface* dst,
     return false;
 }
 
+void GrGLGpu::blendBarrier() {
+    // This shouldn't be called if there is coherent (or no) support for advanced blend equations.
+    SkASSERT(GrDrawTargetCaps::kAdvanced_BlendEquationSupport ==
+             this->caps()->blendEquationSupport());
+    GL_CALL(BlendBarrier());
+}
+
 void GrGLGpu::didAddGpuTraceMarker() {
     if (this->caps()->gpuTracingSupport()) {
         const GrTraceMarkerSet& markerArray = this->getActiveTraceMarkers();