Get menu background image from selected game

src/guiMainMenu.h

@@ -39,6 +39,7 @@ struct MainMenuData
 	// These are in the native format of the gui elements
 	// Generic
 	int selected_tab;
+	std::string selected_game;
 	// Client options
 	std::string servername;
 	std::string serverdescription;
@@ -78,6 +79,7 @@ struct MainMenuData
 	MainMenuData():
 		// Generic
 		selected_tab(0),
+		selected_game("minetest"),
 		// Client opts
 		fancy_trees(false),
 		smooth_lighting(false),