bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Gregory Heytings]

> > Maybe we should measure the increment in MODIFF across the calls to 
> > fontification-functions in handle_fontified_prop, and correct 
> > UNCHANGED_MODIFIED by the same increment when fontification-functions 
> > return?
>
> Hmm... I'm having second thoughts on this idea.  Let me think some more.

I think it would be better to wait for Ihor's feedback before trying to 
fix his problem.

Ihor?