Previously, every call to updateSizes() would update every entity. Now, entities
can be marked as dirty, and updateSizes can be called such that it only affects
dirty entities. I changed a few calls to updateSizes to take advantage of this,
but others can probably be optimized too

This was, previously, explicitly handled with a factor of (1 / (1 - x)).
I just regex'd away these factors, so there are probably a few errors.

By default, the world was set to be 1.1 times as tall as the max height
(or the mean). Now, it can be set to an arbitrary factor.