filter/Internal: rename struct AudioOutput to FilteredAudioOutput

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

test/read_mixer.cxx

@@ -51,7 +51,7 @@ try {
 	EventLoop event_loop;
 
 	Mixer *mixer = mixer_new(event_loop, alsa_mixer_plugin,
-				 *(AudioOutput *)nullptr,
+				 *(FilteredAudioOutput *)nullptr,
 				 *(MixerListener *)nullptr,
 				 ConfigBlock());