Refactor the videorate element to properly separate upstream and downstream
time domains when the `rate` property is set.
The previous implementation had issues with time domain conversions when
changing rates, especially during seeks after which we were basically confusing
input and output timestamps.
This also fixes rate changes as we are now tracking the wanted input timestamps
and not confusing them with the output, so this way we do not drop buffers when
the rate is changed while playing, meaning that the related tests have been
fixed
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9013>
51d43529c3