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

[Gregory Heytings]

> > It is better to do that higher, because not only those places in 
> > decode_mode_spec may wish to access BEGV and ZV.  Even in the mode-line 
> > display there are :eval and :when.
>
> Okay, I'll see what I can do.

Now done in the feature branch, together with a few other improvements.

Both Lars' recipes now work as expected.