automation control: allow single level of push/pop control group

libs/ardour/ardour/automation_control.h

@@ -126,6 +126,7 @@ public:
 
 protected:
 	std::shared_ptr<ControlGroup> _group;
+	std::shared_ptr<ControlGroup> _pushed_group;
 
 	const ParameterDescriptor _desc;
 
@@ -156,6 +157,8 @@ private:
 	*/
 	friend class ControlGroup;
 	void set_group (std::shared_ptr<ControlGroup>);
+	bool push_group (std::shared_ptr<ControlGroup>);
+	bool pop_group ();
 	PBD::ScopedConnection _state_changed_connection;
 	bool _no_session;
 };

libs/ardour/ardour/control_group_member.h

@@ -33,6 +33,8 @@ class LIBARDOUR_API ControlGroupMember
 	   mediated by the ControlGroup, not by operating on the member.
 	*/
 	virtual void set_group (std::shared_ptr<ControlGroup>) = 0;
+	virtual bool push_group (std::shared_ptr<ControlGroup>) = 0;
+	virtual bool pop_group () = 0;
 };
 
 } /* namespace */

libs/ardour/automation_control.cc

@@ -370,6 +370,32 @@ AutomationControl::get_user_string () const
 	return ARDOUR::value_as_string (_desc, get_value());
 }
 
+bool
+AutomationControl::push_group (std::shared_ptr<ControlGroup> cg)
+{
+	if (_pushed_group) {
+		return false;
+	}
+
+	_pushed_group = _group;
+	_group = cg;
+
+	return true;
+}
+
+bool
+AutomationControl::pop_group ()
+{
+	if (!_pushed_group) {
+		return false;
+	}
+
+	_group = _pushed_group;
+	_pushed_group.reset ();
+
+	return true;
+}
+
 void
 AutomationControl::set_group (std::shared_ptr<ControlGroup> cg)
 {