upipe-avfsink: enhancements

- add _get_format(), _set_format() and fix av_format_guess() call
- fix upipe_avfsink->mime leak
- add language metadata
upipe-avcenc: set samples (frame_size) if present
Replace transcode_video example by transcode.

- avformat source/sink (demux/mux)
- optional audio/video transcoding

TODO: fix transcoded audio samplerate/channels