If VO deinterlacing is unavailable, try to insert vf_yadif.
If vf_lavfi is available, actually use vf_yadif from libavfilter. The
libavfilter version of this filter is faster, more correct, etc., so it
is preferred. Unfortunately vf_yadif obviously doesn't support
VFCTRL_GET/SET_DEINTERLACE, and with the current state of the
libavfilter API, it doesn't look like there is any simple way to
emulate it. Instead, we simply insert the filter with a specific label,
and if deinterlacing is to be disabled, the filter is removed again by
label.
This won't do the right thing if the user inserts any deinterlacing
filter manually (except native vf_yadif, which understands the VFCTRL).
For example, with '-vf lavfi=yadif', pressing 'D' (toggle deinterlacing)
will just insert a second deinterlacer filter. In these cases, the user
is supposed to map a command that toggles his own filter instead of
using 'D' and the deinterlace property.
The same applies if the user wants to pass different parameters to the
deinterlacer filters.
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