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

This is implicitly done by the destructor.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

For easier state management inside filter plugins.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

.. and destruct the output object properly in test/run_output.