add an option to make the pinchZoom constrain zoom levels to integers

Changes the default behavior of ol.interaction.PinchZoom to keep
the fractional zoom level after the gesture.
The old behavior of zooming to the next whole-number level can be
enforced with the new parameter `constrainResolution`.

externs/olx.js

@@ -3104,6 +3104,13 @@ olx.interaction.PinchZoomOptions;
  */
 olx.interaction.PinchZoomOptions.prototype.duration;
 
+/**
+ * Zoom to the closest integer zoom level after the pinch gesture ends. Default is `false`.
+ * @type {boolean|undefined}
+ * @api
+ */
+olx.interaction.PinchZoomOptions.prototype.constrainResolution;
+
 
 /**
  * @typedef {{handleDownEvent: (function(ol.MapBrowserPointerEvent):boolean|undefined),

src/ol/interaction/pinchzoom.js

@@ -27,6 +27,12 @@ ol.interaction.PinchZoom = function(opt_options) {
 
   var options = opt_options ? opt_options : {};
 
+  /**
+   * @private
+   * @type {boolean}
+   */
+  this.constrainResolution_ = options.constrainResolution || false;
+
   /**
    * @private
    * @type {ol.Coordinate}
@@ -111,6 +117,7 @@ ol.interaction.PinchZoom.handleUpEvent_ = function(mapBrowserEvent) {
     var map = mapBrowserEvent.map;
     var view = map.getView();
     view.setHint(ol.View.Hint.INTERACTING, -1);
+    if (this.constrainResolution_) {
       var resolution = view.getResolution();
       // Zoom to final resolution, with an animation, and provide a
       // direction not to zoom out/in if user was pinching in/out.
@@ -118,6 +125,7 @@ ol.interaction.PinchZoom.handleUpEvent_ = function(mapBrowserEvent) {
       var direction = this.lastScaleDelta_ - 1;
       ol.interaction.Interaction.zoom(map, view, resolution,
           this.anchor_, this.duration_, direction);
+    }
     return false;
   } else {
     return true;