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

include/gpu/GrSurface.h

Index: include/gpu/GrSurface.h
diff --git a/include/gpu/GrSurface.h b/include/gpu/GrSurface.h
index 52a56659ab11012dd4819f7bdf70eacdd9a51d94..eeb63ad21bfd7f85d125430fb66b25360e6b36b7 100644
--- a/include/gpu/GrSurface.h
+++ b/include/gpu/GrSurface.h
@@ -11,6 +11,7 @@
 
 #include "GrTypes.h"
 #include "GrResource.h"
+#include "SkRect.h"
 
 class GrTexture;
 class GrRenderTarget;
@@ -33,6 +34,12 @@ public:
      */
     int height() const { return fDesc.fHeight; }
 
+    /**
+     * Helper that gets the width and height of the surface as a bounding rectangle.
+     */
+    void getBoundsRect(SkRect* rect) const { rect->setWH(SkIntToScalar(this->width()),
+                                                         SkIntToScalar(this->height())); }
+
     GrSurfaceOrigin origin() const {
         GrAssert(kTopLeft_GrSurfaceOrigin == fDesc.fOrigin || kBottomLeft_GrSurfaceOrigin == fDesc.fOrigin);
         return fDesc.fOrigin;