[Suggestion] Support Audio-Only Playback

[ShiguriVR]

Media that does not contain a video track just throws an error.
It would be nice if the player could handle this scenario by just playing back the audio.

I would also suggest that it be possible to specify an image that appears if audio-only content is being played instead of just a black screen, so that there is some feedback to let people know that there simply isn't any video feed and player isn't broken.