New (& incompatible) screen size hack

This is based on a patch by Charles Perkins:
https://github.com/charlesap/io/commit/cf23cea018c3b469b18091818a61baf0eebf07f6

It's not compatible with the previous hack, so if you were using that
you'll have to update Display.Mod. A small change to Input.Mod is also
need if you want to increase the size of the display.

The normal, unmodified Oberon software still works correctly, of
course.
Fix sign extend undefined behavior

Using a left shift to push a 1 into the sign bit of a signed integer results in
undefined behavior. *sigh*

Use an obscure trick instead.