The optimal way would be to use custom rate based on telecine cadence,
but it's not supported by any driver in practice. So instead, allow
driver to field match and output with duplicated frames, which can be
removed using decimate filter.
Note that in practice all GPU vendors uses single video processor for
all deinterlacing, so IVTC will be done even if mode is not explicitly
selected. At least the field matching part, because correct rate output
is not supported. The main difference is that when selecting more we
output at video frame rate, not field rate.
Kacper Michajłow
c2c081f49a