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

[Eli Zaretskii]

> Date: Fri, 22 Jul 2022 10:16:52 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: Eli Zaretskii <eliz@gnu.org>
> 
> 
> on my laptop, with Emacs from master from a week ago, take respectively 
> 150 seconds and 40 seconds.  With the improvements on master, 1 is 
> instantaneous but 2 still takes about 5 seconds.  Now, with the changes in 
> the feature/long-lines-and-font-locking branch, both are instantaneous. 
> The price of that speedup is that some portions of the buffer will be 
> mis-highlighted, which is unavoidable.

The assertion below can now be violated:

      if (it->narrowed_begv)
        {
          record_unwind_protect (unwind_narrowed_begv, Fpoint_min ());
          record_unwind_protect (unwind_narrowed_zv, Fpoint_max ());
          SET_BUF_BEGV (current_buffer, it->narrowed_begv);
          SET_BUF_ZV (current_buffer, it->narrowed_zv);
          specbind (Qinhibit_widen, Qt);
        }

      val = Vfontification_functions;
      specbind (Qfontification_functions, Qnil);

      eassert (it->end_charpos == ZV);  <<<<<<<<<<<<<<<<<

because of the "narrowing".  (I actually saw this assertion violation
once on the branch, but I cannot reproduce it.)