All typedefs for structs that were present in other header
files have been moved to here. (Except node_t because the
renderer and netcode both define node_t LOL.)
- Refactor significantly (now has its own func, `G_GetNextMap`)
- If gametype uses cups, iterate through cups to find the current level, then grab the next valid level
- If not, get the next valid mapheader for your gametype
- SOC `nextmap`/`marathonnext` is not just deprecated but REMOVED
- Hide the NEXTMAP_ constants again, but leave support dummied out for if we have them publically accessible again
- Also get rid of a bunch of OTHER mapheader stuff we're never gonna use
- NiGHTS Grades? NOPE
- Vanilla titlecard patches? NOPE
- Boss music fadeout/replacement? NOPE
- Select Heading? NOPE
- You've been blocked.
- Don't show maps without lumps on the level select list
- this is me being petty, but making it NOTIMEATTACK in SOC instead of TIMEATTACK so we can reconsider the maps with/without them.
