sky dome/horizon rendering broken

[NaN]

No need to fix anything atm, just a reminder for me, when we eventually switch from joe/jpk to a more capable mesh file format, to adjust skybox geometry so that this workaround can be removed.

In detail:
The old renderer had extra code to center meshes relative to the camera (vertical tracking case) when drawn. The new code ported from gl3 renderer avoids this, but requires the (vertical) center of the mesh to be at origin (z=0). The workaround is doing this when loading the track. It would be cleaner to adjust it in the data though.