audio_format: converted typedef AudioFormat to struct audio_format

Get rid of CamelCase, and don't use a typedef, so we can
forward-declare it, and unclutter the include dependencies.

src/outputBuffer.h

@@ -51,7 +51,7 @@ struct output_buffer {
 	    the buffer becomes non-empty */
 	int lazy;
 
-	AudioFormat audioFormat;
+	struct audio_format audioFormat;
 
 	Notify *notify;
 };