trk importer/exporter

[joevenzon]

So, my original solution was to make the track editor allow you to assign the intermediate control points (1 and 2: either assigning them both to the same point (in 3-vert mode), or to different points (in 4-vert mode)), but that didn't quite produce the results desired. It'd be nice if you fixed this so that the control points get moved so that the surface goes through the intermediate point(s) selected, however in addition: There are tracks (like that part of the nurburgring) where the different parts of the road are meant to be discontinuous left-to-right, so instead of manipulating control points in a bezier, there should really be more than one bezier. I was thinking I'd add support for that when I eventually rewrote the track editor, but who knows when that will happen, so feel free to implement this if you think of a good way to do it.