MidiAlsaSeq: protect concurrent sequencer accesses with mutex

The ALSA sequencer interface is not reentrant and thus we can't queue
MIDI events for output from different threads (which happens with multicore
rendering as processOutEvent() is being called by individual note play
handles). We therefore have to care that we don't call the ALSA sequencer
functions concurrently.

Thanks to Benjamin Kusch for providing a testcase.

Closes #531.

include/MidiAlsaSeq.h

@@ -1,7 +1,7 @@
 /*
  * MidiAlsaSeq.h - ALSA-sequencer-client
  *
- * Copyright (c) 2005-2009 Tobias Doerffel <tobydox/at/users.sourceforge.net>
+ * Copyright (c) 2005-2013 Tobias Doerffel <tobydox/at/users.sourceforge.net>
  *
  * This file is part of Linux MultiMedia Studio - http://lmms.sourceforge.net
  *
@@ -31,6 +31,7 @@
 #include <alsa/asoundlib.h>
 #endif
 
+#include <QtCore/QMutex>
 #include <QtCore/QThread>
 #include <QtCore/QTimer>
 
@@ -130,6 +131,7 @@ private:
 	virtual void run();
 
 #ifdef LMMS_HAVE_ALSA
+	QMutex m_seqMutex;
 	snd_seq_t * m_seqHandle;
 	struct Ports
 	{