Use renderMode image for vehicles layer (performance improvement)

 map.html

@@ -25,6 +25,6 @@
<script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v5.3.0/build/ol.js" integrity="sha384-iQkGyyH4ioz3m+maM3s9MX1Oq67mACa4B9Z3ovUv3Sv37LJ96fx3WnZfLoiC3Wfl" crossorigin="anonymous"></script>
<script tyle="text/javascript" src="lang_pl.js?v9" id="lang_script"></script>
<script tyle="text/javascript" src="common.js?v10"></script>
<script tyle="text/javascript" src="map.js?v22"></script>
<script tyle="text/javascript" src="map.js?v23"></script>
</body>
</html>

 map.js

@@ -814,6 +814,7 @@
        });
        vehicles_layer[type] = new ol.layer.Vector({
            source: vehicles_source[type],
            renderMode: 'image',
        });
        vehicles_last_update[type] = 0;
    });