FxMixer: fix crashes on modifying sends

Added a mutex so that the GUI thread can't change the sends around during the rendering loop.

include/FxMixer.h

@@ -140,6 +140,7 @@ private:
 
 	// make sure we have at least num channels
 	void allocateChannelsTo(int num);
+	QMutex m_sendsMutex;
 
 	void addChannelLeaf( int _ch, sampleFrame * _buf );