More sky fixes for software:

* Thok barriers with slopes now render fine in software
* The solid "sky walls" between different-height thok barriers adjacent to each other are gone. Forever.

src/r_bsp.c

@@ -508,6 +508,9 @@ static void R_AddLine(seg_t *line)
 		SLOPEPARAMS( backsector->f_slope, backf1,  backf2,  backsector->floorheight)
 		SLOPEPARAMS( backsector->c_slope, backc1,  backc2,  backsector->ceilingheight)
 #undef SLOPEPARAMS
+		// if both ceilings are skies, consider it always "open"
+		if (backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum)
+		{
 			if ((backc1 <= frontf1 && backc2 <= frontf2)
 				|| (backf1 >= frontc1 && backf2 >= frontc2))
 			{
@@ -518,10 +521,12 @@ static void R_AddLine(seg_t *line)
 			doorclosed = (backc1 <= backf1 && backc2 <= backf2
 			&& ((backc1 >= frontc1 && backc2 >= frontc2) || curline->sidedef->toptexture)
 			&& ((backf1 <= frontf1 && backf2 >= frontf2) || curline->sidedef->bottomtexture)
-		&& (backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum));
+			//&& (backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum)
+			);
 
 			if (doorclosed)
 				goto clipsolid;
+		}
 
 		// Window.
 		if (backc1 != frontc1 || backc2 != frontc2
@@ -532,6 +537,9 @@ static void R_AddLine(seg_t *line)
 	}
 	else
 #endif
+	{
+		// if both ceilings are skies, consider it always "open"
+		if (backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum)
 		{
 			if (backsector->ceilingheight <= frontsector->floorheight
 				|| backsector->floorheight >= frontsector->ceilingheight)
@@ -543,6 +551,7 @@ static void R_AddLine(seg_t *line)
 			doorclosed = R_DoorClosed();
 			if (doorclosed)
 				goto clipsolid;
+		}
 
 		// Window.
 		if (backsector->ceilingheight != frontsector->ceilingheight

src/r_segs.c

@@ -2032,6 +2032,7 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 	else
 	{
 		// two sided line
+		boolean bothceilingssky = false; // turned on if both back and front ceilings are sky
 
 #ifdef ESLOPE
 		if (backsector->c_slope) {
@@ -2063,10 +2064,11 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 		if (frontsector->ceilingpic == skyflatnum
 			&& backsector->ceilingpic == skyflatnum)
 		{
+			bothceilingssky = true;
 #ifdef ESLOPE
-			worldtopslope = worldhighslope =
+			//worldtopslope = worldhighslope;
 #endif
-			worldtop = worldhigh;
+			//worldtop = worldhigh;
 		}
 
 		ds_p->sprtopclip = ds_p->sprbottomclip = NULL;
@@ -2099,6 +2101,8 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			// ds_p->sprbottomclip = negonearray;
 		}
 
+		if (!bothceilingssky)
+		{
 			if (
 #ifdef ESLOPE
 				worldtopslope < worldhighslope ||
@@ -2125,7 +2129,10 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 				ds_p->tsilheight = INT32_MIN;
 				// ds_p->sprtopclip = screenheightarray;
 			}
+		}
 
+		if (!bothceilingssky)
+		{
 #ifdef ESLOPE
 			if (worldhigh <= worldbottom && worldhighslope <= worldbottomslope)
 #else
@@ -2147,10 +2154,12 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 				ds_p->tsilheight = INT32_MIN;
 				ds_p->silhouette |= SIL_TOP;
 			}
+		}
 
 		//SoM: 3/25/2000: This code fixes an automap bug that didn't check
 		// frontsector->ceiling and backsector->floor to see if a door was closed.
 		// Without the following code, sprites get displayed behind closed doors.
+		if (!bothceilingssky)
 		{
 #ifdef ESLOPE
 			if (doorclosed || (worldhigh <= worldbottom && worldhighslope <= worldbottomslope))
@@ -2200,7 +2209,14 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			markfloor = false;
 		}
 
-		if (worldhigh != worldtop
+		if (bothceilingssky)
+		{
+			// double ceiling skies are special
+			// we don't want to lower the ceiling clipping, (no new plane is drawn anyway)
+			// so we can see the floor of thok barriers always regardless of sector properties
+			markceiling = false;
+		}
+		else if (worldhigh != worldtop
 #ifdef ESLOPE
 			|| worldhighslope != worldtopslope
 			|| backsector->c_slope != frontsector->c_slope
@@ -2226,19 +2242,28 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			markceiling = false;
 		}
 
-		if (backsector->ceilingheight <= frontsector->floorheight ||
+		if (!bothceilingssky)
-		    backsector->floorheight >= frontsector->ceilingheight)
+		{
+#ifdef ESLOPE
+			if ((worldhigh <= worldbottom && worldhighslope <= worldbottomslope)
+			 || (worldlow >= worldtop && worldlowslope >= worldtopslope))
+#else
+			if (backsector->ceilingheight <= frontsector->floorheight
+			 || backsector->floorheight >= frontsector->ceilingheight)
+#endif
 			{
 				// closed door
 				markceiling = markfloor = true;
 			}
+		}
 
 		// check TOP TEXTURE
-		if (worldhigh < worldtop
+		if (!bothceilingssky // never draw the top texture if on
+			&& (worldhigh < worldtop
 #ifdef ESLOPE
 				|| worldhighslope < worldtopslope
 #endif
-			)
+			))
 		{
 			fixed_t texheight;
 			// top texture
@@ -2687,12 +2712,12 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			markfloor = false;
 		}
 
-		if ((
+		if (frontsector->ceilingpic != skyflatnum
+		&& (
 #ifdef ESLOPE
 			frontsector->c_slope ? P_GetZAt(frontsector->c_slope, viewx, viewy) :
 #endif
-			frontsector->ceilingheight) <= viewz &&
+			frontsector->ceilingheight) <= viewz)
-		    frontsector->ceilingpic != skyflatnum)
 		{
 			// below view plane
 			markceiling = false;