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

[Gregory Heytings]

> This reminds me of something I meant to mention -- Stefan M. once 
> proposed that there should be two kinds of narrowing (I think?).  The 
> first is the one that the user sets with `C-x n n', which says "the user 
> is only interested in this bit of the buffer", but programs are 
> "allowed" to remove that restriction when doing stuff (like font 
> locking).  The second type should be a strict one, where modes are not 
> allowed to widen the region.
>
> Looking briefly at Gregory's new branch, it seems like that (sort of) 
> introduces this idea, but in a non-explicit way (i.e., by having an 
> inhibit-widen variable).

Thanks for the idea!  I think this could be useful in other contexts, so 
I'll try to implement that.