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

[Stefan Monnier]

>> Not only more intervals, but also more elements in the `plist`s of
>> each interval.
> Could be, if there are many different properties.  But since we are
> talking about font-lock, there's only one we are talking about: 'face'.

`face` may be the one we care about, but there will be others
(especially in the case you were talking about, i.e. the impact of
non-display-related faces)


        Stefan