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

[Eli Zaretskii]

> Date: Mon, 01 Aug 2022 12:51:47 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, 56682@debbugs.gnu.org, larsi@gnus.org, 
>     monnier@iro.umontreal.ca
> 
> 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...