#702 Wheel optimizations

[Bart]

We could optimize the handling of wheels by:
- One directory for all wheel meshes (speed-dreams-2/data/objects/wheels). This would allow us create more different wheels too.
- Instead of 4 wheel meshes per car, just using one that is flipped and rotated as needed
- Most wheels of various cars are only different by their rims. So it would save space when the rim texture are at car directory and the tyre texture at the wheel directory (speed-dreams-2/data/objects/wheels).