Add handling for drawfullheight flag

Parse drawfullheight flag in UDMF
Draw full height for walls if the linedef has this flag

specs/udmf_zdoom.txt

@@ -141,6 +141,7 @@ Note: All <bool> fields default to false unless mentioned otherwise.
                                   // 13: Portal line.
       revealed = <bool>;          // true = line is initially visible on automap.
       noskywalls = <bool>;        // true = skies are not drawn above or below this line
+      drawfullheight = <bool>;    // true = draw the maximum height instead of the minimum height
 
       health = <int>;             // Amount of hitpoints for this line.
       healthgroup = <int>;        // ID of destructible object to synchronize hitpoints (optional, default is 0)

src/maploader/udmf.cpp

@@ -1078,6 +1078,10 @@ class UDMFParser : public UDMFParserBase
 				Flag(ld->flags, ML_NOSKYWALLS, key);
 				continue;
 
+			case NAME_DrawFullHeight:
+				Flag(ld->flags, ML_DRAWFULLHEIGHT, key);
+				continue;
+
 			case NAME_MoreIds:
 				// delay parsing of the tag string until parsing of the sector is complete
 				// This ensures that the ID is always the first tag in the list.

src/rendering/hwrenderer/scene/hw_walls.cpp

@@ -2023,11 +2023,14 @@ void HWWall::Process(HWDrawInfo *di, seg_t *seg, sector_t * frontsector, sector_
 			float bch1a = bch1, bch2a = bch2;
 			if (frontsector->GetTexture(sector_t::floor) != skyflatnum || backsector->GetTexture(sector_t::floor) != skyflatnum)
 			{
-				// the back sector's floor obstructs part of this wall				
-				if (ffh1 > bch1 && ffh2 > bch2)
+				// the back sector's floor obstructs part of this wall
+				if (!seg->linedef->flags & ML_DRAWFULLHEIGHT)
 				{
-					bch2a = ffh2;
-					bch1a = ffh1;
+					if (ffh1 > bch1 && ffh2 > bch2)
+					{
+						bch2a = ffh2;
+						bch1a = ffh1;
+					}
 				}
 			}
 
@@ -2105,10 +2108,13 @@ void HWWall::Process(HWDrawInfo *di, seg_t *seg, sector_t * frontsector, sector_
 
 		/* bottom texture */
 		// the back sector's ceiling obstructs part of this wall (specially important for sky sectors)
-		if (fch1 < bfh1 && fch2 < bfh2)
+		if (!seg->linedef->flags & ML_DRAWFULLHEIGHT)
 		{
-			bfh1 = fch1;
-			bfh2 = fch2;
+			if (fch1 < bfh1 && fch2 < bfh2)
+			{
+				bfh1 = fch1;
+				bfh2 = fch2;
+			}
 		}
 
 		if (bfh1 > ffh1 || bfh2 > ffh2)

src/utility/namedef.h

@@ -551,6 +551,7 @@ xx(Locknumber)
 xx(Midtex3dimpassible)
 xx(Revealed)
 xx(AutomapStyle)
+xx(DrawFullHeight)
 
 xx(Playercross)
 xx(Playeruse)