cehoyos, who did not agree to the LGPL relicensing, added this in commit
240b743e. The actual implementation of it is already guarded with
HAVE_GPL. The field_dominance field in the option struct won't be
guarded.
We won't keep GPL-only core code forever, so deprecate it as well. To
apply forced deinterlacing, a libavfilter filter can probably be
removed, or we merge this functionality into the --deinterlace option
(without using copyrighted stuff).