tunnel entrances for the NAM tool

[GoaSkin]

The NAM tool makes it possible to build different styles of tunnel entrances for the same network. Currently, one alternative entrance for roads and two for railroads are supported.

If someone is interested in rendering a tunnel entrance - just create a BAT with the following properties:

- tunnels aren't LOTs and do not support props. Everything must be part of the base model - including the road or rail base texture.
- the backside of the model must be rectangular and closed. The slope tile with a tunnel entrance on it does not support transparency. Non-rectangular BATs result in blackholes as surrounding.
- a customized lod is necessary to see the vehicles disappearing in the tunnel. This is more important for train tunnels than for car tunnels.
- the game does not support lightrail, RHW and street tunnels. It is possible to implement some but they are still unused for traffic simulation.

To see how creating custom tunnels work, you may also download the NAM tool and examine the tunnel model plugin file.