Now it's no longer possible to manipulate TID hash from arbitrary location
For example, this prevents linking of destroyed object into the hash
TID member is still public but writing to it is limited to a few very specific cases like serialization and player traveling between levels
https://forum.zdoom.org/viewtopic.php?t=64476
The Map loader may not access any global state at all - everything it can touch must be exchangable.
Furthermore, if we want to sandbox each level, there may be no direct access to any kind of global state whatsoever from the play code.
This should later be done for everything else as well, but the map loader should really be free of global dependencies ASAP.
Also replace TThinkerIterator<AActor> with FThinkerIterator globally because this only adds pointless type checks - with all actor subclasses being scripted this class has become redundant.
Using global variables for this is bad, and it didn't even catch all cases. Now a node build is only considered successful if everything is set up successfully.
# Conflicts:
# src/maploader/maploader.cpp
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 involves passing the level explicitly to many functions. What was done here may seem a bit excessive but at least it covers everything.
Most importantly, the global ActiveThinker pointer has been moved into FLevelLocals and is now getting tracked properly by the level without using dangerous assumptions about how the game organizes its data.
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.
