Fix stairs rendering in Caves and Hell

Stairs can have the same `TileProperties` as floors but do not
always follor the same graphics layout as floors, so we shouldn't
apply optimizations to them.

The only floors affected seem to be:

1. Caves: tile 48 sub-tile 171 frame 461 (TileProperties: None)
2. Hell: tile 46 sub-tile 141 frame 386 (TileProperties: BlocksMissile)

Note that the few broken in pixels in caves are actually incorrectly
attributed to the non-Solid tile, really they should be part of the
solid tile 49.

As there doesn't seem to be a quick and easy way to check if a frame
is part of Stairs, we add a check for BlocksMissile tile property
instead.

This fixes Hell but isn't enough to fix Caves.
To fix Caves, we then add a `BlocksMissile` flag to the broken sub-tile.

Source/engine/render/scrollrt.cpp

@@ -84,7 +84,7 @@ constexpr auto RightFrameDisplacement = Displacement { DunFrameWidth, 0 };
 
 [[nodiscard]] DVL_ALWAYS_INLINE bool IsFloor(Point tilePosition)
 {
-	return !TileHasAny(tilePosition, TileProperties::Solid);
+	return !TileHasAny(tilePosition, TileProperties::Solid | TileProperties::BlockMissile);
 }
 
 [[nodiscard]] DVL_ALWAYS_INLINE bool IsWall(Point tilePosition)

Source/levels/gendung.cpp

@@ -468,6 +468,10 @@ void LoadLevelSOLData()
 		break;
 	case DTYPE_CAVES:
 		LoadFileInMem("levels\\l3data\\l3.sol", SOLData);
+		// The graphics for tile 48 sub-tile 171 frame 461 are partly incorrect, as they
+		// have a few pixels that should belong to the solid tile 49 instead.
+		// Marks the sub-tile as "BlockMissile" to avoid treating it as a floor during rendering.
+		SOLData[170] |= TileProperties::BlockMissile;
 		break;
 	case DTYPE_HELL:
 		LoadFileInMem("levels\\l4data\\l4.sol", SOLData);

Source/levels/reencode_dun_cels.hpp

@@ -19,7 +19,8 @@ struct DunFrameInfo {
 
 	[[nodiscard]] bool isFloor() const
 	{
-		return !HasAnyOf(properties, TileProperties::Solid)
+		// The BlockMissile check is for stairs in L3 and L4, e.g. tile 46 sub-tile 141 frame 386 in L4.
+		return !HasAnyOf(properties, TileProperties::Solid | TileProperties::BlockMissile)
 		    && (microTileIndex == 0 || microTileIndex == 1);
 	}
 	[[nodiscard]] bool isFloorLeft() const { return microTileIndex == 0; }