* Implement renderstyle and transparent actor sprite rendering in automap
* Add "am_advspriterender" CVAR and add invisible actor checks.
* Rename cvar to `am_thingrenderstyles`
currentUILevel is now primaryLevel.
For ZScript, currentVMLevel was added. This is also exported as 'level' and will change as needed.
This also means that no breaking deprecations will be needed in the future, because in order to sandbox a level only 4 variables need to be handled: level, players, playeringame and consoleplayer.
The remaining global variables are not relevant for the level state.
The static 'level' has been mostly removed from the code except some places that still need work.
This made reviewing the code for accessing the global state hard, because the doomdef.h contains mainly constants, this particular item was the only thing in there that represents actual engine state.
This doesn't really write out any info for the pointer, if the level does not match it just errors out.
This is both for quick detection of badly used level data and for automatic restoring of the pointer from the serializer's working level.
This also removed the temporary workarounds in DAutomap and DLevelScript to restore these pointers when a savegame is loaded.
Since the SpawnedThings array is still available when polyobjects are spawned it makes no sense to create an expensive linked list in P_SpawnMapThing.
This can be done far better by scanning through the array again and collect all matching items in a second array.
This was done to make reviewing easier, again because it is virtually impossible to search for the operators in the code.
Going through this revealed quite a few places where texture animations were on but shouldn't and even more places that did not check PASLVERS, although they were preparing some paletted rendering.
A section is a continuous part of a sector or in some case of several nearby continuous parts. For sectors with far away parts multiple sections will be created, especially when they lie in disjoint parts of the map.
This is mainly supposed to cut down on time for linking dynamic lights. Since they need to traverse subsectors to find all touching sidedefs a more coarse data structure that only contains the info needed for this is more suitable. In particular, this does not contain any intra-sector lines, i.e. those with both sides in the same sector.
