bug#56683: 29.0.50; long lines fix doesn't work correctly when lines are truncated

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: andreyorst@gmail.com,  56683@debbugs.gnu.org,  gregory@heytings.org
> Date: Fri, 22 Jul 2022 09:53:33 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Forgot one thing:  There seem to be a place in dispextern.h where
> >> last_visible_x is set to the number 1000000.  That's odd.
> >
> > That's a different kind of "infinity" ;-)
> 
> Aleph1 :-O.

Well, what I meant was that it's used for other purposes than
DISP_INFINITY.