Common vertex format

[kvark]

Considering #25 and #26 to generate more vertex data, we may end up with too much (partially redundant) code if we want the generators to support various combinations like vertex + normal, vertex, vertex + tex-coords.

We might want to consider a single vertex format to be used by all generators, potentially also covering #27. The user should still be able to map the vertex to its own format and drop some of the generated data.

The main problem I see here is the complexity of shared vertex generation. For example, a cube would no longer be able to share any vertices, since each face has a different normal (unless we smooth normals over the cube, which doesn't make much sense).

[kvark]

Another idea is to only generate fat vertices with the Iterator implementation of the generator (thus, non-shared). This is rather straightforward, and then we can use LruIndexer to make shared vertices and indexed polygons from it.