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

include/core/SkMatrix.h

Index: include/core/SkMatrix.h
diff --git a/include/core/SkMatrix.h b/include/core/SkMatrix.h
index f148e390b5cd4930f2d985f61a286cda3748b7cc..71e6b9737b6ddbc7d9f94ec091b5be860e364680 100644
--- a/include/core/SkMatrix.h
+++ b/include/core/SkMatrix.h
@@ -470,6 +470,18 @@ public:
         return this->mapRect(rect, *rect);
     }
 
+    /** Apply this matrix to the src rectangle, and write the four transformed
+        points into dst. The points written to dst will be the original top-left, top-right,
+        bottom-right, and bottom-left points transformed by the matrix.
+        @param dst  Where the transformed quad is written.
+        @param rect The original rectangle to be transformed.
+    */
+    void mapRectToQuad(SkPoint dst[4], const SkRect& rect) const {
+        // This could potentially be faster if we only transformed each x and y of the rect once.
+        rect.toQuad(dst);
+        this->mapPoints(dst, 4);
+    }
+
     /** 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.