Fix selecting root node before button released

In order to test if the button press is within the scene tree, the
Tree's coordinates should be used, not the SceneTreeEditor's.

editor/scene_tree_dock.cpp

@@ -126,7 +126,8 @@ void SceneTreeDock::input(const Ref<InputEvent> &p_event) {
 	Ref<InputEventMouseButton> mb = p_event;
 
 	if (mb.is_valid() && (mb->get_button_index() == MouseButton::LEFT || mb->get_button_index() == MouseButton::RIGHT)) {
-		if (mb->is_pressed() && scene_tree->get_rect().has_point(scene_tree->get_local_mouse_position())) {
+		Tree *tree = scene_tree->get_scene_tree();
+		if (mb->is_pressed() && tree->get_rect().has_point(tree->get_local_mouse_position())) {
 			tree_clicked = true;
 		} else if (!mb->is_pressed()) {
 			tree_clicked = false;