- Cleaner way of switching window sizes at runtime.
- Better handling of divide-by-zero (why an f? why not e for error?)
- Reindent the code (it's a confusing mess of tabs and spaces), and
  reformat it to my personal style.
- Make it play nicer with tiling WMs. If possible, anyway: echinus
  doesn't try to tile it (which is correct, since it won't resize).
  Try some others. Low priority.

- 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.