Fix bug where face groups are not defined

Initialize the face groups array with a single face group in the event
that the .obj file does not explicitly define any.
Add support for n-gons, fix bug

- Remove arguments to MeshFace constructor that did nothing.
- Fix bug related to the fact that Wavefront face indicies are one-based
  and not zero-based.
- Store vertex data in MeshFace differently as a result of n-gon support,
  using the new Vertex class.
Really fix face groups this time.

Properly initialize face groups, no matter if groups are ever defined
in the .obj file or not.