Issue was caused by the way how audio output works from audaspace.
Now made it much closer to what's happening in ffmpeg.c and writeffmpeg.c.
Also fixed issues with incompatible combinations of codecs and formats
in mixdown settings.
Mixdown functionality.
* Mixdown possible via libsndfile and ffmpeg!
* Fixed some ffmpeg deprecation warnings
* Mixdown UI only shows working Container, Codec and Format combinations!
* Minor bugs and warnings fixed