Reset jitter history when frame rate changes.

Reset jitter history when frame rate changes.

This is seemingly especially important when we have input format autodetect
and PAL input rates; it gives us one 59.97 fps frame, then a delay as the
card autodetects and resyncs (might be as much as 30–50 ms; not entirely sure),
and then a steady stream of 50 fps frames. This then causes us to overestimate
the jitter by a lot until we get more than 1000 frames and can reject that
very first event as the outlier it is.