Add blend optimization helpers and use to convert rect draws to clears.

src/gpu/GrBlend.h

Index: src/gpu/GrBlend.h
diff --git a/src/gpu/GrBlend.h b/src/gpu/GrBlend.h
new file mode 100644
index 0000000000000000000000000000000000000000..e70d9457772c30e1f5a5660c5a25b98c58043a1a
--- /dev/null
+++ b/src/gpu/GrBlend.h
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+
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "GrTypes.h"
+#include "GrColor.h"
+
+#ifndef GrBlend_DEFINED
+#define GrBlend_DEFINED
+
+static inline bool GrBlendCoeffRefsSrc(GrBlendCoeff coeff) {
+    switch (coeff) {
+        case kSC_GrBlendCoeff:
+        case kISC_GrBlendCoeff:
+        case kSA_GrBlendCoeff:
+        case kISA_GrBlendCoeff:
+            return true;
+        default:
+            return false;
+    }
+}
+
+static inline bool GrBlendCoeffRefsDst(GrBlendCoeff coeff) {
+    switch (coeff) {
+        case kDC_GrBlendCoeff:
+        case kIDC_GrBlendCoeff:
+        case kDA_GrBlendCoeff:
+        case kIDA_GrBlendCoeff:
+            return true;
+        default:
+            return false;
+    }
+}
+
+GrColor GrSimplifyBlend(GrBlendCoeff* srcCoeff,
+                        GrBlendCoeff* dstCoeff,
+                        GrColor srcColor, uint32_t srcCompFlags,
+                        GrColor dstColor, uint32_t dstCompFlags,
+                        GrColor constantColor);
+
+#endif