avoid spawning creeps directly on top of each other

[nczempin]

trafficstars

in the current level layout, two identical ships spawn on top of each other. let's try to make that situation occur less, by not putting spawners so close to each other, by not spawning the same creep types so close to each other, or by varying the animation slightly so that they don't fly exactly on top of each other (including the "swaying")

[nczempin]

Added some very rudimentary random offsets in 322b22d642f7b98ac5ade7d55c10569cd15396b8, so they no longer fly exactly on top of each other. Sill a lot of improvement opportunities for the overall problem, though