Use texture barriers to read directly from the RT

src/gpu/effects/GrCustomXfermodePriv.h

Index: src/gpu/effects/GrCustomXfermodePriv.h
diff --git a/src/gpu/effects/GrCustomXfermodePriv.h b/src/gpu/effects/GrCustomXfermodePriv.h
index 98883e04fa17bee614e4de4f7d675265dbf22bdf..4f119f94abb0972177fda038f7408b07eeb2f399 100644
--- a/src/gpu/effects/GrCustomXfermodePriv.h
+++ b/src/gpu/effects/GrCustomXfermodePriv.h
@@ -68,6 +68,10 @@ public:
         return false;
     }
 
+    bool willBlendCoherently(const GrDrawTargetCaps& caps) const override {
+        return true;
+    }
+
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;