level_order() uses the first argument to set the loadingbg type. However, in load_level() the first argument is the image location for the background. This implies that if defined we are to use background + bar. This keeps us backward compatible with all existing mods. Strange that I never had the modder specify this before, Hence, this was a bug. All mods ie Contra, Final Fight, GoldenAxeRemake and more now display a background when loading a level.
