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 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.
This function will truncate everything that is larger than LONG_MAX or smaller than LONG_MIN to fit into a long variable, but longs are 32 bit on Windows and 64 bit elsewhere, so to ensure consistency and the ability to parse larger values better use strtoll which does not truncate 32 bit values.
This was to resolve some circular dependencies with the portal code.
The most notable changees:
* FTextureID was moved from textures.h to doomtype.h because it is frequently needed in files that don't want to do anything with actual textures.
* split off the parts from p_maputl into a separate header.
* consolidated all blockmap related data into p_blockmap.h
* split off the polyobject parts into po_man.h
- Viewheight change was delayed: Viewheight must be copied to player
structure as well as the PlayerPawn. Not sure if should actually use
deltaviewheight to spread the change out over a few tics instead of
being instant.
- Viewheight not preserved when travelling: player->viewheight must be
restored from pawn->ViewHeight, because the temporary player set it back
to the default viewheight.
