Implement <resize><drawContents>

<resize><drawContents>[yes|no] configures whether to let the clients
redraw its window content content while resizing.

When <resize><drawContents> is set to no, a multi-rect is shown to
indicate the geometry of the resized window.

include/config/rcxml.h

@@ -118,6 +118,7 @@ struct rcxml {
 	enum tiling_events_mode snap_tiling_events_mode;
 
 	enum resize_indicator_mode resize_indicator;
+	bool resize_draw_contents;
 
 	struct {
 		int popuptime;