libvo: register X11 connection fd in input event system

libvo: register X11 connection fd in input event system

Register the X11 connection fd in the input system so that
mp_input_get_cmd() can immediately wake up and handle keyboard or
other X events. The callback calls vo_check_events() and tells the
input system to handle any input possibly recorded during that. Before
this was done for vo_xv only; this commit generalizes it to all VOs
that call vo_x11_create_vo_window() - those are hopefully ones that
will handle all X events in check_events().

The callback is only kept registered while the vo is properly
configured. At other times calling check_events() would not clear
pending input and so could lead to a busy loop.