Make Gr clear take a rect for a partial-clear

Review URL: http://codereview.appspot.com/4442093/



git-svn-id: http://skia.googlecode.com/svn/trunk@1203 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/include/GrContext.h b/gpu/include/GrContext.h
index 3112112..214fc05 100644
--- a/gpu/include/GrContext.h
+++ b/gpu/include/GrContext.h
@@ -282,9 +282,11 @@
     // Draws
 
     /**
-     *  Clear the entire render target, ignoring any clips
+     * Clear the entire or rect of the render target, ignoring any clips.
+     * @param rect  the rect to clear or the whole thing if rect is NULL.
+     * @param color the color to clear to.
      */
-    void clear(GrColor color);
+    void clear(const GrIRect* rect, GrColor color);
 
     /**
      *  Draw everywhere (respecting the clip) with the paint.