Scheduled events are now in event time order, so schedule_run simply takes the top
event off the heap and runs it (if we're polling, rather than having received a signal
that the next event is due to be run, we check first that we have passed the scheduled
time).
This should provide performance benefits as we are no longer trawling the entire list
of scheduled events (potentially hundreds) every time an event is signalled or the
schedule list is polled.
svn path=/trunk/netsurf/; revision=11703
doesn't remove the need for INTUITICKS messages as we still need to break out for page
reflow (which doesn't use the scheduler), but it should make animations more accurate.
svn path=/trunk/netsurf/; revision=9648
