bug#56682: feature/improved-locked-narrowing 9dee6df39c: Reworked locked narrowing.

[Stefan Monnier]

> Also note that previously, I'd never have suggested that redisplay code
>  changes the value of font-lock-dont-widen.

It would be ugly for redisplay code to mess with font-lock data like
`font-lock-dont-widen`.  Instead, it should let-bind its own variable to
indicate that the long-lines code has currently installed a narrowing,
and `font-lock.el` can decide whether to pay attention to that var.


        Stefan