title: OpenLayer 3 - GeoJSON - feature is null
body: <p>I'm trying to get country string by coordinates, but feature is null.</p>
  
  <p>Why feature is null? how could I fix it?</p>
  
  <p>I tested with onclick <strong>event.pixel</strong> and it does returns feature but then I use <strong>map.getPixelFromCoordinate</strong> to get <strong>pixel</strong> feature becomes null</p>
  
  <pre><code>var vectorLayer = new ol.layer.Vector({
      source: new ol.source.Vector({
          url: 'https://openlayers.org/en/v4.6.5/examples/data/geojson/countries.geojson',
          format: new ol.format.GeoJSON()
      })
  });
  
  var map = new ol.Map({
      layers: [ vectorLayer],
      target: 'map',
      view: new ol.View({
          center: [0, 0],
          zoom: 1
      }),
      logo:false
    });
  </code></pre>
  
  <p>this does not work</p>
  
  <pre><code>map.once('postrender', function() {
      var pixel = map.getPixelFromCoordinate([-0.0508, 51.5160]);
  
      var feature = map.forEachFeatureAtPixel(pixel, function(feature) { 
          return feature;
      });
  
      console.log("Country:"+feature.get("name"));
  });
  </code></pre>