bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> Even if we decide to disable truncate-lines in such buffers, we won't 
> prevent users from enabling it.

That's correct, but if they consciously decide to shoot themselves in the 
foot, that's not Emacs' responsibility anymore.

> And if and when they enable it, we'd still like to give them the best 
> performance we can.  So speeding up the performance when truncate-lines 
> is enabled is still a worthy goal, although if that is disabled in 
> long-line buffers, that goal becomes less important.

I fully agree with that.

> I'm trying to speed up these situations as much as possible, and I think 
> it will serve us well even if we decide to turn off truncate-lines in 
> long-line buffers.

Thanks!