FR: collision meshes

Overlaps some with #4112 but from a different angle.

I desperately want to define collision meshes that are not any detail level and are not rendered. This should be specified at the pof level, not the table level, perhaps with a property on the collision mesh or the detail0 mesh, but I'm not too firm on the implementation details.

The needs of a visual mesh at any detail level are pretty different from an ideal collision mesh, and besides building efficient collision meshes this would be useful in some abstract ship ideas I have as well.

Related to this we may wish to explore the possibility of adding some collision primitive data to the pof, such as being able to define multiple bounding spheres for sphere-mesh collisions instead of the main ship radius.