This is so that individual audio players can share the same mutex as the
mixer. When they have their own, it's apparently quite easy to
accidentally introduce deadlocks. Particularly when the audio player is
shut down.
This also adapts our default implementation MixerImpl to handle the newly
added flags properly.
Now we do not need to set the sound volume for all types to 0, in case we want
to mute them, but instead just set the mute flag for all types to true. This
allows engines to be a bit more agonstic about mute support, when it comes to
volume options etc. since they can just setup any volume they like, but are
still muted (and thus will not break muting anymore).
MIDI sound is of course not affected by this.
The RateConverter::flow result was never used, pipe it through
Channel::mix to MixerImpl::mixCallback, so backends can decide
if they want to waste cpu cycles while playing empty buffers.
