Fix a bug where `Media#extras` property is lost when handling updates to `playlist` (`MPV_EVENT_PROPERTY_CHANGE`)

[default-anton]

Steps to reproduce

// Create a playlist where each media has unique clip_id in extras
final playlist = Playlist([
  Media(
    'file.mp4',
    extras: {'clip_id': 'file.mp4 1'},
    start: Duration.zero,
    end: Duration(seconds: 10),
  ),
  Media(
    'file.mp4',
    extras: {'clip_id': 'file.mp4 2'},
    start: Duration(seconds: 10),
    end: Duration(seconds: 20),
  ),
]);

await player.open(playlist);

for (final media in player.state.playlist.medias) {
  debugPrint('media: ${media.extras}');
}

// Prints:
// media: {clip_id: file.mp4 2}
// media: {clip_id: file.mp4 2}

Observe that every instance of Media has extras of the last media in the playlist.