ffmpeg backend always report duration 0 for webm file

1. try to play any .webm file
2. MediaPlayer.duration is always 0
3. console has the warning "AVStream duration -9223372036854775808 is invalid. Taking it from the metadata"
4. From my understanding, you should always look for duration from the AVFormatContext::duration or AVFormatContext::metadata, AVStreamContext may not have the duration metadata at all