bug#56393: Actually fix the long lines display bug

[Eli Zaretskii]

> Date: Mon, 18 Jul 2022 17:07:26 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, larsi@gnus.org, 56393@debbugs.gnu.org
> 
> Thanks for the detailed explanation.  It's clear that computing that 
> information would largely outweigh its potential benefit, so I'll have to 
> live without that potential optimization.

If you had a way to do this, for how many characters would you need to
have this info?  (I presume you want it for characters before point,
but how many?)