Make players fit within their hit/collision box

This makes the players look a bit like slendermen, but I swear, I used a real biological model! I think the lack of legs makes them look extra weird.

Also refactored the Entity code a bit. Now we don't store HalfExtents or CenterOffset on the client anymore, we instead get them sent over by the server when the entity is created. This ensures the hitbox/collision box that the client displays always matches what the server is using, and paves the way for adding different types of entities in the future (although the server logic still needs some work before that point... but one step at a time).

Fixes #127.