player: add disc-menu state holder and overlay support

The new player/discnav.c owns the player-side state for the in-disc
menu UI, shared between stream_bluray and stream_dvdnav. Once per
playloop iteration it pulls the stream's nav state and

- pushes the BD menu plane to the OSD (OSDTYPE_DISC_MENU) and the DVD
  highlight state to the dvd_subtitle decoders,
- resyncs playback when the disc VM jumped on its own, detected
  through the stream's discontinuity counter,
- mirrors the disc's current title into demuxer->edition,
- force-selects a dvd_subtitle track while a DVD menu is up, so the
  menu graphics decoded by sd_lavc reach the OSD, and
- enables the {discnav} input section while a menu is interactive.

The per-file state is reset on every new playlist entry.

meson.build

@@ -170,6 +170,7 @@ sources = files(
     'player/client.c',
     'player/command.c',
     'player/configfiles.c',
+    'player/discnav.c',
     'player/external_files.c',
     'player/loadfile.c',
     'player/main.c',