All it does is "sometimes" close the mixer, but that is also done in
the destructor.

Fixes shutdown memory leak.

This is implicitly done by the destructor.

Fixes shutdown memory leak.

The output plugin shall decide whether to insert silence or do nothing
at all.  The ALSA output plugin has already implemented this.
Inserting silence is not necessary or helpful for some plugins, and
may even hurt them (e.g. "recorder").

Eliminates unnecessary mutex locks.

Closes #181

Wired to Filter::Flush().

Closes #153

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

By API contract, this parameter must not be zero.

Adds missing checks to several plugins.