Avbin ffmpeg

This PR takes care of making the ffmpeg and avbin plugins more similar.

Main changes to avbin:
- now supports looping
- provides meta data (same kind as ffmpeg)
- does provided its length

I found that the reported duration is different for ffmpeg and avbin. Subsequently the reported length of the video can differ one frame between ffmpeg and avbin. I think we can live with that. It seems better IMO than reporting a length of inf.

Further, it seems that avbin always gives a black image for the first 5 frames. ffmpeg does not. Maybe have look at that at some point. See #25.