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

[Stefan Monnier]

>> The wholelines problem did not kick in because of PPS nor `syntax-ppss`
>> but because of font-lock (which then called `syntax-ppss` which then
>> called PPS).
> If it's font-lock that forces syntax-ppss to examine the whole huge
> line, then what is your proposal for avoiding that which doesn't
> involve some more-or-less arbitrary restrictions on the part of the
> buffer that can be examined by syntax-ppss?

The use of `syntax-wholeline-max` in
`font-lock-extend-region-wholelines` supposedly fixed this problem since
it changed `font-lock` so it doesn't ask `syntax-ppss` to compute the
whole line/buffer.


        Stefan