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

[Stefan Monnier]

> It is, indeed.  But here we are talking about big files (say 1 GB).  In that
> case (and in that case only) it is much better, from a user viewpoint, to
> wait say 20 seconds before the file is opened and being at that point able
> to freely move through the file, instead of waiting only 6 seconds, and then
> having to wait another 10 seconds between two motion commands like M-> C-p.
> In fact, no user expects that a 1 GB file would open instantaneously.

The problem will re-occur if you change something at the beginning of
the buffer and then go back to the end of the buffer.

I think something like `font-lock-defer` which displays the text
unfontified until we had time to apply the fontification should make the
1GB case much more usable (tho currently, it's probably not quite good
enough).


        Stefan