Fix parasitic sky-hack vertical planes

[team-eternity]

Fix parasitic sky-hack vertical planes

[ioan-chera]

Fixed but not fully. Parasitic sky vertical planes appear where they don't in MBF.

[ioan-chera]

Fixed another discovered bug (in commit d0213a). But the main issue still remains. The difference between being in a normal sector and being above a fake ceiling is caused by this check in R_2S_Normal and equivalently in R_2S_Sloped:

   mark = (seg.frontsec->lightlevel != seg.backsec->lightlevel ||
           seg.line->linedef->portal ||
           seg.frontsec->heightsec != -1 ||
           seg.frontsec->heightsec != seg.backsec->heightsec ||
           seg.frontsec->midmap != seg.backsec->midmap ||
           (seg.line->sidedef->midtexture && 
            (seg.line->linedef->extflags & EX_ML_CLIPMIDTEX)));

heightsec becomes valid and makes the sky-hack linedef always render.

[ioan-chera]

Fixed by removing the seg.frontsec->heightsec != -1 check. Hopefully no other bugs arise from this.