options: disable --field-dominance for WIP LGPL mode, also deprecate

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).

DOCS/interface-changes.rst

@@ -35,6 +35,7 @@ Interface changes
       the reference white level instead of in cd/m^2
     - renamed the TRCs `st2084` and `std-b67` to `pq` and `hlg` respectively
     - the "osd" command is deprecated (use "cycle osd-level")
+    - --field-dominance is deprecated (no replacement)
  --- mpv 0.25.0 ---
     - remove opengl-cb dxva2 dummy hwdec interop
       (see git "vo_opengl: remove dxva2 dummy hwdec backend")

options/options.c

@@ -436,8 +436,11 @@ const m_option_t mp_opts[] = {
     OPT_CHOICE("video-aspect-method", aspect_method, UPDATE_IMGPAR,
                ({"hybrid", 0}, {"bitstream", 1}, {"container", 2})),
 
+#if HAVE_GPL
     OPT_CHOICE("field-dominance", field_dominance, UPDATE_IMGPAR,
-               ({"auto", -1}, {"top", 0}, {"bottom", 1})),
+               ({"auto", -1}, {"top", 0}, {"bottom", 1}),
+               .deprecation_message = "no replacement"),
+#endif
 
     OPT_SUBSTRUCT("vd-lavc", vd_lavc_params, vd_lavc_conf, 0),
     OPT_SUBSTRUCT("ad-lavc", ad_lavc_params, ad_lavc_conf, 0),