Surround and MTR in sndio backends

[gavv]

Requires: #694, #695.

• [ ] Sox:
  • [ ] SoxSink, SoxSource: surround: correct mapping for alsa, coreaudio
  • [ ] SoxSink, SoxSource: surround: fallback for rest
  • [ ] SoxSink, SoxSource: mtr
• [ ] Wav:
  • [ ] WavSink: write WAVEX extension header with channel mask
  • [ ] WavSink: surround
  • [ ] WavSink: mtr
  • [ ] WavSource: read channel mask from dr_wav and pass it to ChannelSet
  • [ ] WavSource: surround
  • [ ] WavSource: mtr
• [ ] Sndfile:
  • [ ] SndfileSink, SndfileSource: surround: #693
  • [ ] SndfileSink, SndfileSource: mtr
• [ ] Pulseaudio
  • [ ] surround
  • [ ] mtr