Fix breakage from commit bea5681f

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

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

Avoid accessing FilteredAudioFormat attributes directly.

Configure the AudioFormat after opening the encoder, because only now
we know the final format.

This order will be necessary for proper AudioFormat initialization,
because the encoder may change the format.

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