- Cleaner way of switching window sizes at runtime.

- Long range possibility: rewrite the display/input stuff to use SDL2,
  so we can scale arbitrarily. Probably this would mean it'd work on
  Wayland too, though I don't much care about that.

- Find a real TI Programmer II calculator somewhere and compare.
  Probably there's room for a more accurate (si|e)mulation.