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

[Gregory Heytings]

> I think we'll want to decouple the two because they aren't quite the 
> same (e.g. in many cases the time taken by `syntax-ppss` doesn't depend 
> on the length of lines, whereas the time taken by `font-lock-keywords` 
> rues does not depend on the total buffer size).

That's what I initially thought, too.  Now I tend to think that it would 
be better to not decouple these two cases, because it would be too 
expensive to distinguish the four possible cases, namely:

1. "normal buffers"

2. non-large buffers with long lines

3. large buffers without long lines

4. large buffers with long lines

It is probably better to group the three last cases in a single group, and 
to use the same optimization techniques for these three cases.