output/Control: unlock mutex during expensive operations

src/output/Control.cxx

@@ -226,6 +226,7 @@ AudioOutputControl::Open(const AudioFormat audio_format,
 	const bool open2 = open;
 
 	if (open2 && output->mixer != nullptr) {
+		const ScopeUnlock unlock(mutex);
 		try {
 			mixer_open(output->mixer);
 		} catch (...) {

src/output/Thread.cxx

@@ -501,5 +501,6 @@ AudioOutputControl::StartThread()
 {
 	assert(command == Command::NONE);
 
+	const ScopeUnlock unlock(mutex);
 	thread.Start();
 }