Add RailIndex function for objects

[leezer3]

https://github.com/leezer3/OpenBVE/issues/1009

Unfinished WIP.

Current progress:

The creation of objects (in master) uses a large number of parameters, which are often passed through two or three levels of function calls. This refactors them into a single helper struct. Next job is to save the rail index in the animated object, and provide an appropriate function accessor for them.

Risks:

Minor, however this changes a lot of places. If we manage to get the starting / ending distances bugged somewhere, this is likely to lead to visibility gliches. Needs careful testing....