Script spawned beams (via sexp) do not have their graphics correctly loaded

If a beam spawned via script, via a runSEXP `beam-create`, is not used elsewhere traditionally in the mission it's graphics will not be loaded, and its sections will be invisible. `beam_fire` specifically checks for beams which are 'not used', and loads their graphics, but `beam-create` calls `preload_turret_change_weapon` which marks the weapon as being used, despite its graphics not being loaded.