Fix some ways to create inconsistent Viewport sizes

In the editor, it was possible to set the size of a `SubViewport` even
in cases where a parent `SubViewportContainer` had stretch enabled.

This PR disables editing a `SubViewport.size` while the parent disallows
it and it makes necessary adjustments during `NOTIFICATION_ENTER_TREE`.

scene/gui/subviewport_container.h

@@ -58,6 +58,7 @@ public:
 	virtual void unhandled_input(const Ref<InputEvent> &p_event) override;
 	void set_stretch_shrink(int p_shrink);
 	int get_stretch_shrink() const;
+	void recalc_force_viewport_sizes();
 
 	virtual Size2 get_minimum_size() const override;