Calling meta_later_add() or meta_later_remove() from within a
META_LATER_BEFORE_REDRAW callback ended up being a no-op, because of
how run_repaint_laters() was fiddling with the laters list. (This
resulted in a crash in window.c:idle_calc_repaint(), which assumed it
would only be called when a certain queue was non-empty, but was
getting called anyway because of a failed meta_later_remove() call.)
Fix this by having run_repaint_laters() work on a copy of the laters
list instead, and add refcounting to MetaLater so that removing a
later that run_repaint_laters() hasn't gotten to yet won't cause
problems.
https://bugzilla.gnome.org/show_bug.cgi?id=642957
cd7a968093