Material rendering

Materials can now be rendered. In the current state, this simply means that each type of collision material has a .mat file associated with them, which is loaded at the start of the simulation. The .mat file is currently just a simple material with no texture, just a flat color, but it will be easy to change the texture of the materials at a later time.

See some example use of the materials in the video below:

Note that changing material also changes friction/energyloss values but since those aren't added yet the simulation breaks