Auto-discovery of multi-line font-lock regexps

[Stefan Monnier <monnier@iro.umontreal.ca>]

While reading the "regexp does not work as documented" thread, an idea
came to me: we could have an idle task that takes the font-lock regexps
and instead of applying them directly, only uses them to find matches
that span multiple lines and mark them with the
`font-lock-multiline' property.

The idea is that the font-lock-multiline works OK to preserve multiline
matches, so the real difficulty is in making sure we discover
them correctly.  Sometimes we do by happenstance, sometimes we do
because the major-mode was careful to make it work (which is far from
trivial), but often we just don't.  Having such a background loop would
be very helpful.  Its job can easily be stopped at any time, so it
shouldn't introduce long latencies.


        Stefan