Fix handheld maps not displaying in Minetest 5.5.0

The function dynamic_add_media() was changed in incompatible ways in
several minor versions of Minetest, breaking the display of handheld
maps in Minetest 5.5.0. This patch makes handheld maps display there.

The function was blocking with one argument in Minetest 5.3. It was also
blocking in Minetest 5.4, but took an additional argument for a function
to execute once the media had been received. Calling dynamic_add_media()
with a single argument had been deprecated; a function that did nothing
was provided in mcl_maps to satisfy the changed argument requirements.

In Minetest 5.5, dynamic_add_media() was changed to non-blocking. This
introduced a race condition in mcl_maps, where a client often tried to
display a map before it had received the map texture from the server.

Opening an issue on the Minetest issue tracker led to it being closed in
about 20 minutes: <https://github.com/minetest/minetest/issues/11997>

mods/ITEMS/mcl_maps/init.lua

@@ -147,11 +147,23 @@ function mcl_maps.load_map(id)
 	local texture = "mcl_maps_map_texture_" .. id .. ".tga"
 
 	if not loaded_maps[id] then
-		loaded_maps[id] = true
+		if not minetest.features.dynamic_add_media_table then
-		dynamic_add_media(map_textures_path .. texture, function() end)
+			-- minetest.dynamic_add_media() blocks in
+			-- Minetest 5.3 and 5.4 until media loads
+			loaded_maps[id] = true
+			dynamic_add_media(map_textures_path .. texture, function() end)
+		else
+			-- minetest.dynamic_add_media() never blocks
+			-- in Minetest 5.5, callback runs after load
+			dynamic_add_media(map_textures_path .. texture, function()
+				loaded_maps[id] = true
+			end)
+		end
 	end
 
-	return texture
+	if loaded_maps[id] then
+		return texture
+	end
 end
 
 function mcl_maps.load_map_item(itemstack)