Change DABG to vertical mirroring

[NovaSquirrel]

If I can do this, I can retain the UNROM+WRAM option, which is currently out because of needing to be able to change mirroring mid-game. I don't think I would need to do much; for every queued write, I would just write it to both nametables, and I would need to copy the rendered level to the second nametable on level start.

As a bonus this means DABG would actually work on the VS System.