player/command: make track-list/N/ff-index optional

The `ff-index` is just a index of in the track order, it doesn't carry
much meaning and may be wrong or unavailable when other than lavf
demuxer is used. Additionally some tracks may be virtual like DOVI EL
track or merged image tile grid. In which case those track are not
related to lavf at all.

No deprecation period. It was never guaranteed to match FFmpeg anyway,
so anyone who has a hard dependency on this property should reconsider.
It's not really possible to provide backward compatibility given the
ambiguity of this index. It's better to make it unavailable than making
mismatch. For user it's clear when it's not available, but not possible
to know when it's wrong.

player/command.c

@@ -2115,7 +2115,8 @@ static int get_track_entry(int item, int action, void *arg, void *ctx)
         {"main-selection", SUB_PROP_INT(order), .unavailable = order < 0},
         {"external-filename", SUB_PROP_STR(track->external_filename),
                         .unavailable = !track->external_filename},
-        {"ff-index",    SUB_PROP_INT(track->ff_index)},
+        {"ff-index",    SUB_PROP_INT(track->ff_index),
+                        .unavailable = track->ff_index == -1},
         {"hls-bitrate", SUB_PROP_INT(track->hls_bitrate),
                         .unavailable = !track->hls_bitrate},
         {"program-id",  SUB_PROP_INT(sh && sh->num_program_ids ? sh->program_ids[0] : -1),