Modified the height map function to create a mesh for a subset of the
given height map.
Added the ability to retrieve entities of a given category.
Added categories to components.
Removed the position attribute from the Text model.
Upgraded the QuadTree to work with any type of model and work in 3D
worlds.
Added an OctTree.
Implemented supporting functions.
Replaced AABB2 with a new Square model.
Added functions for constructing bounding volumes from meshes.
Interface improvements.