source: rework volume handling

- drop the 'virtual_' prefix from s->virtual_volume since we don't
  distuingish between reference and real volumes for sources

- introduce an accuracy for source volumes: if the hardware can control
  the volume "close enough" don't necessarily adjust the rest in
  software unless it is beyond a certain threshold. This should save a
  little bit of CPU at the expensive of a bit of accuracy in volume
  handling.

- other minor cleanups

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