Changes:
Implement software surround downmixing (fix#796).
Fix a crash when no audio renderer were created when stopping emulation.
NOTE: This PR also disable support of 5.1 surround on the OpenAL backend as we cannot detect if the hardware directly support it. (the downmixing applied by OpenAL on Windows is terribly slow)
* audout:u: Implement SetAudioOutVolume and GetAudioOutVolume
- Implementation of `audout:u` SetAudioOutVolume and GetAudioOutVolume (checked with RE).
- Add Get and Set for Volume into audio backends.
- Cleanup of all audio backends to follow the `IAalOutput` structure and .NET standard.
- Split OpenAL backend into 2 files for consistency.
* Address comments
* Fix the volume calculation