If we ever want to refactor the global level data these must not reference the 'level' variable.
The main parts of the map loader cannot use this information, because it can only be created after running the node builder, so it got its own set of index functions instead.
This thing made sense when GZDoom and ZDoom were separate projects to avoid having to change some core files for the added options.
Now, with only 3 ones remaining, one for FraggleScript and two for Extradata the overhead here is just too high. The 3 variables can just be moved to level_info_t without carrying along this much baggage.
