Fix direntSearch bug

Should return the relative path to the files, not just the filenames.
Don't compute inertia of static meshes

It makes no sense and would trigger an assert(0).
Update materials, meshes, add test kart

The test kart is just a white box with "Pretend this is a kart" on the
front, but it's usefull to test new things without breaking the existing
karts.