aout: rework stereo-mode and add mix-mode
cf. the original discussion here: https://mailman.videolan.org/pipermail/vlc-devel/2020-July/135357.html
- Restores the VLC 2.x behavior for --stereo-mode: use it only for stereo content.
- Add a new option: --mix-mode: allow the user to force an output channel configuration
- Move headphones (Binaural) handling from --stereo-mode to --mix-mode
- Transform the spatialaudio "audio filter" to an audio converter, automatically loaded via i_chan_mode (like the dolby audio converter)
Here are some screenshots of stereo/mix mode menus:
Stereo mode menu when playing stereo:
Mix mode menu when playing 5.1:
Mix mode menu when playing Ambisonics:
