Screen overlay mode

[teuler]

Issues

1. When only one screen is attached, the overlay window appears somewhere "outside" screen one ... needs to be fixed.
2. int_screen_width_pix = 0 in [Stage] needs to be 0 as if for full screen mode, otherwise activating bool_use_screen_overlay causes an error of some kind ...
3. The GUI for changing the .ini file with respect to the overlay screen setting seems not to work yet; int_screen_index_gui in [Overlay] is weird when !=0 ... ??