3.1.0

Added selective backface culling. All triangles are visible,
unless otherwise specified in an input file, such as 'tunnb.cld'
from directory 'data', which looks like this:

2
1 56 v 1 0 5
57 72 i 1 0 5

Line 1 is obvious, line 2 means that triangles 1...56 are visible
from point M(x=1,y=0,z=5) and line 3 means that triangles 57...72
are not visible from point M(x=1,y=0,z=5); x, y and z are
coordinates in the local system of the object.

Unlike in version 3.0.0, when you get to the end of the track, you
are teleported to the other end.


4.0.0

Added background images (all pictures were taken by me), changed
sound, modified all tracks, changed some parameter values.
