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 enabled. When first seen by a spawn shooter (which does not always have Romero's head attached, but usually does), the player will hear "To win the game you must kill me, John Romero" played backwards, which sounds something like "grrrrrr reh ooh seye, j'imbre selfetiay dombrin deus".

The shooter is a Doom Thing, but it has no sprite and thus 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 "aroused" sound, but never shoots any cubes.

Monsters which may be spawned:
 * Imp
 * Demon
 * Spectre
 * Cacodemon
 * Hell Knight
 * Baron of Hell
 * Mancubus
 * Arachnotron
 * Revenant
 * Pain Elemental
 * Arch-Vile