Add devicePixelRatio option to ol.Map constructor

src/objectliterals.jsdoc

@@ -25,6 +25,9 @@
  * @typedef {Object} olx.MapOptions
  * @property {ol.Collection|Array.<ol.control.Control>|undefined} controls
  *     Controls initially added to the map.
+ * @property {number|undefined} devicePixelRatio The ratio between physical
+ *     pixels and device-independent pixels (dips) on the device. If `undefined`
+ *     then it gets set by using `window.devicePixelRatio`.
  * @property {ol.Collection|Array.<ol.interaction.Interaction>|undefined} interactions
  *     Interactions that are initially added to the map.
  * @property {Array.<ol.layer.Base>|ol.Collection|undefined} layers Layers.

src/ol/map.js

@@ -157,6 +157,13 @@ ol.Map = function(options) {
 
   var optionsInternal = ol.Map.createOptionsInternal(options);
 
+  /**
+   * @private
+   * @type {number}
+   */
+  this.devicePixelRatio_ = goog.isDef(options.devicePixelRatio) ?
+      options.devicePixelRatio : ol.BrowserFeature.DEVICE_PIXEL_RATIO;
+
   /**
    * @private
    * @type {goog.async.AnimationDelay}