- Track a saved mouse pointer location from the previous frame so we can detect

- Track a saved mouse pointer location from the previous frame so we can detect
  motion.

- Track a timeout value so we can optionally hide the mouse pointer after
  some user specified timeout period.

- in doMouseMotion() always update m.x and m.y even if we return early because
  pui wanted the event.  Without this, we can't reliably detect motion vs.
  inactivity.

- in _update_mouse() add a dt parameter so we can decriment the timeout value
  in "real" time.

- in _update_mouse() optionally hide the mouse pointer if m.timeout goes to
  zero.  Restore the pointer (and the timeout counter) if the mouse is moved.