Ignore layer filter for unmanaged layers

To make unmanaged vector layers work like the removed ol.FeatureOverlay,
the layer filter for ol.Map#forEachFeatureAtPixel needs to ignore unmanaged
layers.

src/ol/layer/layer.js

@@ -148,10 +148,13 @@ ol.layer.Layer.prototype.handleSourcePropertyChange_ = function() {
 
 /**
  * Sets the layer to be rendered on a map. The map will not manage this layer in
- * its layers collection, and the layer will be rendered on top. This is useful
- * for temporary layers. To remove an unmanaged layer from the map, use
- * `#setMap(null)`. To add the layer to a map and have it managed by the map,
- * use {@link ol.Map#addLayer} instead.
+ * its layers collection, layer filters in {@link ol.Map#forEachLayerAtPixel}
+ * will not filter the layer, and it will be rendered on top. This is useful for
+ * temporary layers. To remove an unmanaged layer from the map, use
+ * `#setMap(null)`.
+ *
+ * To add the layer to a map and have it managed by the map, use
+ * {@link ol.Map#addLayer} instead.
  * @param {ol.Map} map Map.
  * @api
  */