Fixed two bugs in SW-only clip mask generation

http://codereview.appspot.com/6306086/



git-svn-id: http://skia.googlecode.com/svn/trunk@4290 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 9389499..71263d4 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -342,8 +342,11 @@
      * 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.
+     * @param target if non-NULL, the render target to clear otherwise clear
+     *               the current render target
      */
-    void clear(const GrIRect* rect, GrColor color);
+    void clear(const GrIRect* rect, GrColor color, 
+               GrRenderTarget* target = NULL);
 
     /**
      *  Draw everywhere (respecting the clip) with the paint.