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.

src/output/plugins/AlsaOutputPlugin.cxx

@@ -64,7 +64,7 @@ class AlsaOutput final
 
 	friend struct AudioOutputWrapper<AlsaOutput>;
 
-	AudioOutput base;
+	FilteredAudioOutput base;
 
 	Manual<PcmExport> pcm_export;