Changed: Do not spread objlinks over two-sided linedefs when the midd…

…le material on the back side of the spread direction completely fills the gap between floor and ceiling. Fixes the issue seen in Heretic E1M2 where the Undead Warriors hidden behind a one-way wall in sector #83 were sometimes visible from the other side. This also fixes the problem of dynamic lights spreading under similar circumstances.

doomsday/engine/portable/src/p_objlink.c

@@ -359,6 +359,12 @@ boolean RIT_ContactFinder(linedef_t* line, void* data)
         return true;
     }
 
+    // Don't spread if the middle material completely fills the gap between
+    // floor and ceiling (direction is from dest to source).
+    if(Rend_DoesMidTextureFillGap(line,
+        dest == line->L_backsector? BACK : FRONT))
+        return true;
+
     // Calculate 2D distance to line.
     vtx = line->L_v1;
     distance = ((vtx->V_pos[VY] - params->objPos[VY]) * line->dX -