creeps sometimes appear to survive a fireball

[plexsoup]

sometimes it looks like a creep doesn't die. That may be two creeps overlapping and only one takes the hit, or it may be a creep spawning another before moment of impact. Either way it feels like a bug. Maybe we should give the bullets a short-lived AoE on impact, to exterminate secret bugs.

[plexsoup]

I added some avoidance behaviour to them, so they're less likely to overlap.