Fix arpeggio to work better with the new way to handle note offsets

Ok, not really related to memory management, but was something that needed doing and it's easier to test things when things work properly

include/NotePlayHandle.h

@@ -31,6 +31,7 @@
 #include "PlayHandle.h"
 #include "track.h"
 #include "MemoryManager.h"
+#include <QtCore/QAtomicInt>
 
 class InstrumentTrack;
 class NotePlayHandle;
@@ -335,6 +336,7 @@ private:
 	static NotePlayHandleList s_nphCache;
 	static NotePlayHandleList s_available;
 	static QMutex s_mutex;
+	static QAtomicInt s_availableIndex;
 };