Destructible terrain debug text now only appears on destructible terrain instead of all terrain.  Also minor code formatting.
Debug text can now be toggled on/off with the backquote/grave key (key with tilde).
Renamed and renumbered USEREVENTS.
Added spawn points to Arena, added max_num_monsters attribute to Arena, added MONSTER_SPAWN_EVENT and timer, added debug text for current number of monsters spawned.