wb: fix incorrect handling of skirmisher.

fixes #3014

src/whiteboard/manager.cpp

@@ -796,7 +796,9 @@ void manager::save_temp_move()
 			//either switch over to a plain route for planned moves, or mark it correctly
 			pathfind::marked_route route;
 			route.steps = move_arrow->get_path();
-			route.move_cost = path_cost(route.steps,*u);
+			// path_cost() is incomplete as it for example doesn't handle skirmisher, we let the move action generate the costs on it own.
+			// route.move_cost = path_cost(route.steps,*u);
+			route.move_cost = -1;
 
 			sa.queue_move(turn, *u, route, move_arrow, std::move(fake_units_[i]));
 		}

src/whiteboard/move.cpp

@@ -352,6 +352,10 @@ void move::apply_temp_modifier(unit_map& unit_map)
 		unit = &*unit_it;
 	}
 
+	if (route_->move_cost == -1) {
+		// TODO: check_validity also calls pathfind::mark_route(get_route().route), optimize/refactor this to only call that once.
+		route_->move_cost = pathfind::mark_route(get_route().route).move_cost;
+	}
 	//Modify movement points
 	DBG_WB <<"Move: Changing movement points for unit " << unit->name() << " [" << unit->id()
 			<< "] from " << unit->movement_left() << " to "