After 4faeb127, the maximum time allowed for an update to happen
is calculated as:

  max_render_time_allowed = refresh_interval - 1000 * sync_delay;

However, extremely small refresh intervals -- that come as consequence
to extremely high refresh rates -- may fall into an odd numerical range
when refresh_interval < 1000 * sync_delay. That would give us a negative
time.

To be extra cautious about it, add another sanity check for this case.

Change suggested by Jasper St. Pierre.

!363