speed up maprect for scale+trans case

include/core/SkMatrix.h

Index: include/core/SkMatrix.h
diff --git a/include/core/SkMatrix.h b/include/core/SkMatrix.h
index 6738a0eb69b17a1fa0a953acdd4c446815c1b1cc..d6d029597f14598dfeb4140767227b7ae7b506f2 100644
--- a/include/core/SkMatrix.h
+++ b/include/core/SkMatrix.h
@@ -561,6 +561,12 @@ public:
         this->mapPoints(dst, 4);
     }
 
+    /**
+     *  Maps a rect to another rect, asserting (in debug mode) that the matrix only contains
+     *  scale and translate elements. If it contains other elements, the results are undefined.
+     */
+    void mapRectScaleTranslate(SkRect* dst, const SkRect& src) const;
+    
     /** Return the mean radius of a circle after it has been mapped by
         this matrix. NOTE: in perspective this value assumes the circle
         has its center at the origin.