trainmap section theme

[Vap0r1ze]

This adds a new "Sections" theme for the train map, where you can see the sections of the train network on the map in the same way as if you were holding a train signal.

Previews:

[Vap0r1ze]

I'm willing to make this feature accessible differently, it's currently accessible via CClient.TrainMapTheme.SECTION. If y'all think it's best to ship this as a widget toggle, or ship it alongside sprites for signals let me know.

It's worth noting theres an issue that still needs to be addressed where rails on a lower y-level can render on top of rails on a higher y-level, though I plan on fixing this before merge

[JensenJ]

Not that I have any authority at all in the matter, so feel free to ignore. But personally, I think a toggle widget would be quite useful and my preferred way of accessing this feature, as it seems the kind of thing that you'd want to quickly turn on and off.

Switching themes through the ingame config viewer may be cumbersome for some, not to mention some people may not know you can switch the theme at all. This may be out of scope for this specific PR, but maybe a selection for all themes on the map screen may be worthwhile?

As for showing signals on the map, I think it may get a little cluttered at tight junctions, etc. If you were to do this I'd just be mindful of that. However, it would be quite nice to have that functionality.