bug#13675: Extremely slow redisplay when lines are very long

[Mark Diekhans]

Is there anyone working on the long-line display speed issue?

It is especially bad with font lock mode.  SIGUSR2 doesn't help.
Twice in the last week I have had to kill emacs because it had
been locked up for more than 10 minutes.

If the fix is really complex, it would be much better emacs
refuse to display the file than force killing it.

thanks!!