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

[Stefan Monnier]

> redisplay are different.  When we find one such interval, it is
> guaranteed to have changes only in the text properties that are of
> interest to redisplay, but the search could take more time if there
> are many text properties that are not interesting, because there are
> more intervals to check.

Not only more intervals, but also more elements in the `plist`s of
each interval.


        Stefan