Fix Viewport being visible after leaving tree

When a SubViewport leaves the tree, it is still displayed in its parent
SubViewportContainer until the next redraw.
This PR makes sure, that the parent gets redrawn immediately.

This also fixes the visibility problem when a SubViewport is added as
child of a SubViewportContainer.

scene/gui/subviewport_container.cpp

@@ -227,6 +227,18 @@ void SubViewportContainer::unhandled_input(const Ref<InputEvent> &p_event) {
 	}
 }
 
+void SubViewportContainer::add_child_notify(Node *p_child) {
+	if (Object::cast_to<SubViewport>(p_child)) {
+		queue_redraw();
+	}
+}
+
+void SubViewportContainer::remove_child_notify(Node *p_child) {
+	if (Object::cast_to<SubViewport>(p_child)) {
+		queue_redraw();
+	}
+}
+
 PackedStringArray SubViewportContainer::get_configuration_warnings() const {
 	PackedStringArray warnings = Node::get_configuration_warnings();
 

scene/gui/subviewport_container.h

@@ -44,6 +44,9 @@ protected:
 	void _notification(int p_what);
 	static void _bind_methods();
 
+	virtual void add_child_notify(Node *p_child) override;
+	virtual void remove_child_notify(Node *p_child) override;
+
 public:
 	void set_stretch(bool p_enable);
 	bool is_stretch_enabled() const;