Stairs: Add backface culling to stair meshes

If backface culling is not specified for a tile in 'images' it is set to true.
Slabs already have backface culling due to being defined as nodeboxes (which
are then converted to meshnodes).

mods/stairs/init.lua

@@ -46,12 +46,24 @@ end
 -- Node will be called stairs:stair_<subname>
 
 function stairs.register_stair(subname, recipeitem, groups, images, description, sounds)
+	local stair_images = {}
+	for i, image in ipairs(images) do
+		if type(image) == "string" then
+			stair_images[i] = {
+				name = image,
+				backface_culling = true,
+			}
+		elseif image.backface_culling == nil then -- override using any other value
+			stair_images[i] = table.copy(image)
+			stair_images[i].backface_culling = true
+		end
+	end
 	groups.stair = 1
 	minetest.register_node(":stairs:stair_" .. subname, {
 		description = description,
 		drawtype = "mesh",
 		mesh = "stairs_stair.obj",
-		tiles = images,
+		tiles = stair_images,
 		paramtype = "light",
 		paramtype2 = "facedir",
 		is_ground_content = false,