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.
