Fixed: Enhanced missing Material selection algorithm to not pic the s…

…ubsitute from an adjacent Surface if the back height is considered "closed" (fixes teleport flat on the side of the secret teleporter in DOOM2::MAP04).

doomsday/engine/portable/src/r_world.c

@@ -1651,7 +1651,8 @@ static material_t* chooseFixMaterial(sidedef_t* s, segsection_t section)
         sector_t* backSec = s->line->L_sector(sid^1);
         surface_t* suf;
 
-        if(backSec)
+        if(backSec && ((section == SEG_BOTTOM && frontSec->SP_floorheight < backSec->SP_floorheight && frontSec->SP_ceilheight  > backSec->SP_floorheight)) ||
+                       (section == SEG_TOP    && frontSec->SP_ceilheight  > backSec->SP_ceilheight  && frontSec->SP_floorheight < backSec->SP_ceilheight))
         {
             suf = &backSec->SP_plane(section == SEG_BOTTOM? PLN_FLOOR : PLN_CEILING)->surface;
             if(suf->material && !R_IsSkySurface(suf))