Improve monster behaviour
monsters: don't cast light when following a player
This made FPS too low for players using integrated graphics when pokenades are spammed. Also was a bit weird, they're not objectives or powerups.
monsters: ensure they can't be stuck when spawned
If too stuck to fix the spawn fails.
monsters: stop following player if they spectate in FFA gametypes
Fixes #2705
In teamplay the DIFF_TEAM check already handled this.
monsters: minor movement refactor
Should probably update angles field according to destination before using it to update velocity.
monsters: try to avoid danger
Also makes them avoid falling off platforms and into holes, and try to find another path, when NOT chasing an enemy. This makes them more useful for guarding an area, especially on space maps.
monsters: improve following and wandering behaviour
Makes monsters wander around the player they're following when nearby, and wander towards the player using the follow update interval when further away. This prevents them clustering tightly around the player and increases their chance of overcoming trivial obstacles.
Fixes trying to walk into walls.