Add an Overlay in the export-map example

examples/export-map.css

@@ -0,0 +1,6 @@
+.overlay {
+  background-color: yellow;
+  border-radius: 6px;
+  padding: 4px;
+  white-space: nowrap;
+}

examples/export-map.html

@@ -8,5 +8,8 @@ docs: >
 tags: "export, png, openstreetmap"
 ---
 <div id="map" class="map"></div>
+<div style="display: none;">
+  <div class="overlay" id="null">Null Island</div>
+</div>
 <a id="export-png" class="btn btn-default"><i class="fa fa-download"></i> Download PNG</a>
 <a id="image-download" download="map.png"></a>

examples/export-map.js

@@ -1,5 +1,6 @@
 import Map from '../src/ol/Map.js';
 import View from '../src/ol/View.js';
+import Overlay from '../src/ol/Overlay.js';
 import GeoJSON from '../src/ol/format/GeoJSON.js';
 import {Tile as TileLayer, Vector as VectorLayer} from '../src/ol/layer.js';
 import {OSM, Vector as VectorSource} from '../src/ol/source.js';
@@ -25,11 +26,17 @@ const map = new Map({
   })
 });
 
+map.addOverlay(new Overlay({
+  position: [0, 0],
+  element: document.getElementById('null')
+}));
+
+
 // export options for html-to-image.
 // See: https://github.com/bubkoo/html-to-image#options
 const exportOptions = {
   filter: function(element) {
-    return element.className.indexOf('ol-control') === -1;
+    return element.className ? element.className.indexOf('ol-control') === -1 : true;
   }
 };