Request: Recoloring for more feature types (namely, track types)

[oberhümer]

So I've been doing a little work on NuTracks lately (release coming soon) and looked into the possibility of autogenerating some of the track sprites via palette color replacement - but to my knowledge track types are one of the few features with neither sprite layouts that can apply recoloring nor a recoloring callback. Now I could write a script that calls Gimp, ImageMagick or whatever - and if anyone has a way to specifically swap by palette index, let me know - but it would be nice to just use OpenTTD's built-in recoloring capabilities. The other use case besides generating several styles of track from a single image would be applying/leaving terrain-specific sprite modifications.