Callback: Build probability

Probability of reversing rail vehicle on build (162)

This callback assigns rail vehicles a random chance of being built facing forward or backward (reversed) when purchased.

Return a value between 0...100 (decimal) to indicate the probability of being reversed.

The same random-reverse behaviour can be implemented without the callback, by randomly switching to reversed sprites in the vehicle's varaction 2 chain, but this is inefficient as it typically causes sprites to be duplicated in the grf.

This callback is available from 14  OpenTTD 14 and above.