Track Debugging Improvement

[joevenzon]

The track collision uses the actual model geometry, so it'd be pretty easy to provide an option to only draw collideable geometry.

In general in the past we were stingy with which geometry was allowed to be collideable for performance reasons, but with NaN's latest physics changes the collision code is wayyyy faster (probably because bullet is doing a lot of the brunt work now) so we could stand to make much more of the track flagged collideable.

I can guarantee that the road surface on all of the tracks is collideable, though, so in the mean time just stay on that.