Pretty standard HD38820 game.  Plate 0 on the VFD is always on, which
is the maze graphics.

There's a 4*4 button matrix with diodes on everything to prevent
ghosting if multiple buttons are down.

The 1 player / 2 player / demo switch defaults to "2 player"
if the "1 player" or "Demo" outputs are low.  (buttons all pull
inputs high when pressed).