Model: Adds more general support of the Wavefront .obj file format. We still only support triangular faces (the format itself allows for arbitrary polygonal faces), however, we now support vertex normals and texcoords supplied with the object, and the format actually allows for separate indexing into each of those attribute lists, so those must be tracked and represented separately by Model (both loading and when converting to Mesh). We do not support any of the more complex features of the file format like curves, free-form surfaces, and the various material description options (separate file load and file format).