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

[Gregory Heytings]

> > diff --git a/src/xdisp.c b/src/xdisp.c
> > index 8a19b3bda9..9574d06bd5 100644
> > --- a/src/xdisp.c
> > +++ b/src/xdisp.c
> > @@ -3472,6 +3472,9 @@ init_iterator (struct it *it, struct window *w,
> >                          &it->bidi_it);
> >          }
> >
> > +      if (current_buffer->long_line_optimizations_p)
> > +       it->narrowed_begv = 0;
> > +
>
> Sorry, I wrote that this is OK, but it isn't: if init_iterator is called 
> with 'struct it' that was already initialized by a previous call to 
> 'reseat', the above will nuke the narrowing.
>
> So we need something more complicated.  ATM I don't see how to solve 
> this without manually initializing narrowed_begv before the first call 
> to init_iterator or start_display.  Hmm...

Hmmm...  So I wasn't completely wrong when I wasn't sure it was TRT 😉 
It's a chicken-and-egg problem.  On the other hand, even if we nuke the 
narrowing there, it will be recomputed two lines later, which should be 
okay.