Monster spawner

Spawn shooters shoot skull cubes that spawn monsters where they land. In the process, the cubes can telefrag, killing anything on the landing spot, even a player with god mode on. When up against a spawner (which does not always have Romero's head with it, but usually does), the player will hear "To win the game you must kill me, John Romero" played backwards, and it sounds like "grrrrrr reh ooh seye, j'imbre selfetiay dombrin deus".

The shooter is a Doom Thing, but it has no sprite and thus it is not visible.

The skull cubes land at spawn spots placed around the map. The spawn spots are used in rotation, in the order they are defined in the THINGS lump in the WAD file. If there are no spawn spots, the spawn shooter still makes its wakeup sound, but never shoots any cubes.

Monster list:
 * Imp
 * Demon
 * Spectre
 * Cacodemon
 * Hell Knight
 * Baron of Hell
 * Mancubus
 * Arachnotron
 * Revenant
 * Pain Elemental
 * Arch-Vile