This fixes two issues, both of which resulted from the timers-wait
period not being re-calculated after idle-observers were executed:
- If timers were added from an idle observer then they could fire long
after they were due (only when/if the next mpv event arrives).
- Idle observers don't execute in zero time, and the wait period for
the next timer was implicitly extended by the idle observers
execution time (because it was calculated before the idle observers).
This commit ensures that if idle-observers were executed, then the
timers wait period is re-calculated, which solves both issues.
5b40db385a