[Rewrite] Fix the audio backend

[FreeApp2014]

In current state the audio backend is absolutely broken...

The backend takes whatever the file tells it to for the channel count and always duplicates the left channel on both speakers as if the file was mono

If the file is not stereo, the configuration of the audio unit format is such, that the audio unit will never be able to start, taking down the whole process

To address these problems, the following must be done:

• Always initialize the backend in stereo to get rid of the crash on non-standard configurations
• Prepare for multi-track support: use the internal Brstm struct fields for retrieving track information and addressing channels using track_lchannel_id and track_rchannel_id
• Count on track_num_channels instead of full file channel count

[FreeApp2014]

Fixed in b98f8cd