Fix disable_3d=yes -Wunused-variable errors

(cherry picked from commit f25b0578462e94d6b0bacb160ff886d4e3320d85)

scene/main/viewport.cpp

@@ -391,9 +391,11 @@ void Viewport::_notification(int p_what) {
 
 			if (physics_object_picking && (to_screen_rect == Rect2() || Input::get_singleton()->get_mouse_mode() != Input::MOUSE_MODE_CAPTURED)) {
 
+#ifndef _3D_DISABLED
 				Vector2 last_pos(1e20, 1e20);
 				CollisionObject *last_object = NULL;
 				ObjectID last_id = 0;
+#endif
 				PhysicsDirectSpaceState::RayResult result;
 				Physics2DDirectSpaceState *ss2d = Physics2DServer::get_singleton()->space_get_direct_state(find_world_2d()->get_space());