Draft: Expose nextgametype, skipstats and nextmapoverdrive as Lua globals and add keepcutscene

Game:

Adds boolean keepcutscene

Forces the game play post-level cutscene despite skipstats.

Alongside it, this merge also adds an additional flag to Custom Exit linedef. (TMEF_KEEPCUTSCENE, 4)

(Needs to be tested, reason why this Merge request is a draft. I couldn't find anything on my hands with post level cutscenes. They are rare... and I kind of don't have time to create one myself. I am still on hotfixing Styles...)

Lua API:

global	accessibility	type
nextmapoverdrive	READ ONLY	int
nextgametype	READ ONLY	int
skipstats	READ ONLY	int
keepcutscene	READ ONLY	bool

Function Update:

G_SetCustomExitVars([int nextmap, [int skipstats], [int nextgametype], [boolean keepcutscene])

Note: int nextgametype parameter is already part of the function. Just wiki is outdated in this case.

Test script:

getCustomExitVars.lua

I will test the keepcutscene and make sure everything works tomorrow when I return.