Add road vehicle state to UpdateViewport()->GetImage() var whitelist.

Minor refactoring of var check.

src/road_cmd.cpp

@@ -1751,6 +1751,7 @@ static VehicleEnterTileStatus VehicleEnter_Road(Vehicle *v, TileIndex tile, int
 			RoadVehicle *rv = RoadVehicle::From(v);
 			if (rv->frame == RVC_DEPOT_STOP_FRAME &&
 					_roadveh_enter_depot_dir[GetRoadDepotDirection(tile)] == rv->state) {
+				rv->cur_image_valid_dir = INVALID_DIR;
 				rv->state = RVSB_IN_DEPOT;
 				rv->vehstatus |= VS_HIDDEN;
 				rv->direction = ReverseDir(rv->direction);