provide mechanism for "drop all slaves" for VCA master

libs/ardour/ardour/slavable.h

@@ -67,8 +67,10 @@ class LIBARDOUR_API Slavable
 	mutable Glib::Threads::RWLock master_lock;
 	std::set<uint32_t> _masters;
 	PBD::ScopedConnection assign_connection;
+	PBD::ScopedConnectionList unassign_connections;
 
 	int do_assign (VCAManager* s);
+
 };
 
 } // namespace ARDOUR

libs/ardour/ardour/vca.h

@@ -60,6 +60,8 @@ class LIBARDOUR_API VCA : public Stripable,
 	XMLNode& get_state();
 	int set_state (XMLNode const&, int version);
 
+	PBD::Signal0<void> Drop; /* signal to slaves to drop control by this VCA */
+
 	/* Slavable API */
 
         bool slaved_to (boost::shared_ptr<VCA>) const;

libs/ardour/slavable.cc

@@ -115,10 +115,13 @@ Slavable::do_assign (VCAManager* manager)
 void
 Slavable::assign (boost::shared_ptr<VCA> v)
 {
+	assert (v);
 	Glib::Threads::RWLock::WriterLock lm (master_lock);
 	if (assign_controls (v) == 0) {
 		_masters.insert (v->number());
 	}
+
+	v->Drop.connect_same_thread (unassign_connections, boost::bind (&Slavable::unassign, this, v));
 }
 
 void