output/Internal: add attribute request.audio_format

Make in_audio_format accessible only from within the OutputThread, and
add a new one for inter-thread communication.

src/output/OutputThread.cxx

@@ -134,7 +134,7 @@ inline void
 AudioOutput::Open()
 {
 	assert(!open);
-	assert(in_audio_format.IsValid());
+	assert(request.audio_format.IsValid());
 
 	fail_timer.Reset();
 
@@ -144,6 +144,8 @@ AudioOutput::Open()
 		/* still no luck */
 		return;
 
+	in_audio_format = request.audio_format;
+
 	bool success;
 
 	{