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

[Gregory Heytings]

> So to try to debug any remaining speed issues with font-lock, it would 
> be great to eliminate other sources of slow display first. In particular 
> ones coming from a low-level subsystem I cannot as easily 
> benchmark/debug/etc as Lisp code.
>
> The setting
>
> (setq long-line-threshold nil syntax-wholeline-max most-positive-fixnum)
>
> indeed makes all the speed issues come back, including the 
> aforementioned sluggishness.

If you want to see only the slowdowns cause by font locking, just comment 
out the "if (current_buffer->long_line_optimizations_p)" in 
xdisp.c:handle_fontified_prop.