Put x first in all coordinate pairs.

Put x first in all coordinate pairs.

This is the universal convention _almost_ everywhere. When writing
actual curses programs I've sometimes reversed it, because curses's
API puts line before column. But (a) ratatui is the sensible way round
in the first place; (b) if my plan in this code is to localise the
curses-type library as hard as possible into one module, then I
wouldn't want to allow its conventions to leak into the rest of the
code _anyway_.

So, here we speak (x,y) like normal people, and since ratatui happens
to be sensible, we do so _everywhere_; if for some reason we had to
work with actual curses, we'd localise the swap to tui.rs.