Use texture barriers to read directly from the RT

src/effects/SkArithmeticMode_gpu.h

Index: src/effects/SkArithmeticMode_gpu.h
diff --git a/src/effects/SkArithmeticMode_gpu.h b/src/effects/SkArithmeticMode_gpu.h
index f4d12d4f5a2c50def1b0462e4d1adce201b0d023..ddfc04354de5a4d6d6cea8a59c103fb30f236bc8 100644
--- a/src/effects/SkArithmeticMode_gpu.h
+++ b/src/effects/SkArithmeticMode_gpu.h
@@ -85,6 +85,10 @@ public:
         return false;
     }
 
+    bool willBlendCoherently(const GrDrawTargetCaps& caps) const override {
+        return true;
+    }
+
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;