Memory Leak for Objects "Falling to Infinity"

[1bardesign]

There appears to be infinitely-increasing memory usage for an object falling into infinity, eventually leading to a crash as the luajit memory limit is hit, and lagging things up in the meantime as the gc has a bigger workload.

Expected Cause:
I believe this is caused by the broadphase structure accumulating empty buckets as the entity falls forever into the void.
It'd be better if the buckets cleaned themselves up when/as they were emptied.

Workaround:
Can be worked around by defining world bounds and killing OoB objects.

(discovered around the same time as #47 but only recently reminded to report it by @tesselode )