Make hitboxes work again

I was planning to fix (but didn't) #127, but the first step was just to get the darn hitboxes working again. Since I had to change a bit of code, and I have a lot of other stuff to do this weekend, I figured I would just propose this as a pull request.

Big change: PlayerMesh now rotates itself, objects added to PlayerEntity are no longer rotated automatically (although they can add rotation themselves if they want to).