This is for the sake of command.c and the "deinterlace" option/property.
Instead of forcing certain "better" defaults when inserting yadif,
change the actual "yadif" defaults.
I pondered not changing vf_yadif, and instead adding a trivial "yadif-
auto" wrapper filter, which would merely have different defaults. But
thinking about it, it doesn't make any sense for "deinterlace" to have
different defaults from vf_yadif, with vf_yadif having the "worse"
defaults. If someone wants the old behavior, the old behavior can be
forced in a backward and forward compatible way by setting the
suboptions.
Fixes#2539 (kind of).