Handle XDND mouse rects synchronously

Handle XDND mouse rects synchronously

* src/xterm.c (x_dnd_send_position): Record event X and Y for
consumption by the XdndStatus handler.  Ignore mouse rects if
waiting for status.
(x_dnd_send_leave): Clear pending DND event.
(handle_one_xevent): When handling XdndStatus, check if the
pending event is contained in the new mouse rect.