Export map with points/polygons

[rondauva]

request from multiple users to be able to create an image file of a neatline exhibit at a specific zoom level with all map annotations visible at that zoom level.

basically a screenshot of a map view; no requests for data from the body text, title text, or description fields.

[waynegraham]

When we finish migrating to OpenLayers 3, this should exceptionally simple; something like

var map = new ol.Map({
  layers: [
    new ol.layer.Tile({
      source: new ol.source.OSM()
    })
  ],
  target: 'map',
  controls: ol.control.defaults({
    attributionOptions: /** @type {olx.control.AttributionOptions} */ ({
      collapsible: false
    })
  }),
  view: new ol.View({
    center: [0, 0],
    zoom: 2
  })
});

var exportPNGElement = document.getElementById('export-png');

if ('download' in exportPNGElement) {
  exportPNGElement.addEventListener('click', function(e) {
    map.once('postcompose', function(event) {
      var canvas = event.context.canvas;
      exportPNGElement.href = canvas.toDataURL('image/png');
    });
    map.renderSync();
  }, false);
} else {
  var info = document.getElementById('no-download');
  /**
   * display error message
   */
  info.style.display = '';
}