Implement removal of unused verticies.

Mesh now has a (fairly primative) method for removing unused and
duplicated verticies. This means its possible to use conbined
geometry without duplicate verticies.

In my test scene, this halved the number of verticies in the combined
mesh.

There are plenty of other optimisations that could be implemented. Some
that would be nice to have but I don't have time for:

* Removal of internal faces in combined meshes
* Optimise triangle order so GL_TRIANGLE_STRIP or the like can be used