Map Renderer|Fixed: No so-called "fake contrast" for middle surfaces …

…of two-sided lines

The id tech 1 software renderer does not apply a line-angle derived
light level delta to the middle surfaces of two-sided lines.

Todo for later: Make this behavior optional with a new side flag.

doomsday/engine/portable/src/render/rend_main.c

@@ -1753,10 +1753,17 @@ static boolean rendHEdgeSection(HEdge* hedge, SideDefSection section,
         SideDef* frontSide = HEDGE_SIDEDEF(hedge);
         float deltaL, deltaR, diff;
 
-        // Do not apply an angle based lighting delta if this surface's material
-        // has been chosen as a HOM fix (we must remain consistent with the lighting
-        // applied to the back plane (on this half-edge's back side)).
-        if(frontSide && isTwoSided && section != SS_MIDDLE && (surface->inFlags & SUIF_FIX_MISSING_MATERIAL))
+        /**
+         * Do not apply an angle based lighting delta if:
+         * a) This surface is the middle wall section of a two-sided line. The
+         *    id tech 1 software renderer does not apply this effect to such
+         *    surfaces. @todo Make this a sidedef flag?
+         * b) This surface's material has been chosen as a HOM fix (we must remain
+         *    consistent with the lighting applied to the relative back plane (on
+         *    this half-edge's back side)).
+         */
+        if(frontSide && isTwoSided &&
+           (section == SS_MIDDLE || (surface->inFlags & SUIF_FIX_MISSING_MATERIAL)))
         {
             deltaL = deltaR = 0;
         }