Letterboxing

[oatmealine]

working on letterboxing (related #21); idea is to make 4:3-only modfiles playable in 16:9 by adding letterboxing
current known issues:

• afts don't respect letterboxing (capturing the entire screen, then stretching the aft sprite of that across the letterboxed space)
• non-configurable
• the actual letterboxes don't show up (cus they're in the screengameplay overlay; worth moving it to overlayscreen, which is why refactors of it are also here temporarily)
• judgments are misplaced