(This is just a safety commit before doing some more extensive behind-the-scenes refactoring.)
Notable changes here:
* use the same logic for determining whether a 3D floor is 'below' or 'above' the actor as all the other functions.
* removed the broken code which tried to detect whether an actor was touching a steep slope. Better use P_LineOpening to find the correct planes and store the results.
* improved detection whether the slopes on both sides of a plane are identical, using the same data as for steep slope detection.
- some consolidation in p_map.cpp. PIT_CheckLine and PIT_FindFloorCeiling had quite a bit of redundancy which has been merged.
- čontinued work on FMultiBlockLinesIterator. It's still not completely finished.
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
