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.

This call cannot fail, but needs cleanup if others fail.  By moving it
to the end, we eliminate some cleanup.

Moved code from AudioOutputControl::InternalOpen2().

Circumvent filter_new() with a fake ConfigBlock.

struct AudioOutput should not know or care about multi-threading.

Was broken by commit 8a86460b

Closes #87