All backgrounds are now handled by a seperate routine.
This will allow more fine handling without a ton of special cases everywhere.
Creates an absolutely nasty diff because all menu_t have been updated in one blast 🥹
For the things that DO need to draw outside of GS_MENU (M_DrawOptionsCogs, the Wrongwarp), the behaviourflag MBF_DRAWBGWHILEPLAYING has also been added.