* Rate events over at least one second
EventsPerSecond.eps() divided the event count by the time since start(),
which can be a few milliseconds right after a restart. Frames buffered
during an ffmpeg restart then report as 100+ fps, and the same happens to
the detector fps. Use a window of at least one second.
* Keep sub-second windows consistent
Floor the divisor at the window length when the window is shorter than a
second, so a caller with a sub-second window still gets its true rate.
