input: Add comment explaining why pointer isn't freed when device is removed

When the last pointer is removed from a seat, the pointer struct is
intentionally kept.  This has some interesting side effects, so I've
documented it here so people like me don't errantly assume it's a bug.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>

src/input.c

@@ -2226,6 +2226,11 @@ weston_seat_release_pointer(struct weston_seat *seat)
 
 		weston_pointer_reset_state(pointer);
 		seat_send_updated_caps(seat);
+
+		/* seat->pointer is intentionally not destroyed so that
+		 * a newly attached pointer on this seat will retain
+		 * the previous cursor co-ordinates.
+		 */
 	}
 }