- renamed the flag bits for sector_t::MoreFlags, so that they are easier to distinguish from sector_t::Flags.

- precalculate if a sector's floor and ceiling plane overlap. This avoids rechecking this for each single call of hw_FakeFlat.
- vertices must be marked dirty every time they change after map setup. That means that ChangePlaneTexZ must do this as well, because it cannot rely on interpolation taking care of it.
- Having a 'dirty' argument for SetPlaneTexZ's ZScript version makes no sense. If the value changes from the script side the vertices must always be marked to be recalculated.

src/p_setup.cpp

@@ -4232,7 +4232,7 @@ void P_SetupLevel (const char *lumpname, int position)
 			{
 				if ((ff->flags & (FF_EXISTS | FF_RENDERPLANES)) == (FF_EXISTS | FF_RENDERPLANES))
 				{
-					s.MoreFlags |= SECF_IGNOREHEIGHTSEC;	// mark the heightsec inactive.
+					s.MoreFlags |= SECMF_IGNOREHEIGHTSEC;	// mark the heightsec inactive.
 				}
 			}
 		}