Use a private variable to cache the default style array

See #1815
Fixes #4063

src/ol/style/style.js

@@ -239,48 +239,45 @@ ol.style.createStyleFunction = function(obj) {
 };
 
 
+/**
+ * @type {Array.<ol.style.Style>}
+ * @private
+ */
+ol.style.defaultStyle_ = null;
+
+
 /**
  * @param {ol.Feature} feature Feature.
  * @param {number} resolution Resolution.
  * @return {Array.<ol.style.Style>} Style.
  */
 ol.style.defaultStyleFunction = function(feature, resolution) {
-  var fill = new ol.style.Fill({
+  // We don't use an immediately-invoked function
-    color: 'rgba(255,255,255,0.4)'
-  });
-  var stroke = new ol.style.Stroke({
-    color: '#3399CC',
-    width: 1.25
-  });
-  var styles = [
-    new ol.style.Style({
-      image: new ol.style.Circle({
-        fill: fill,
-        stroke: stroke,
-        radius: 5
-      }),
-      fill: fill,
-      stroke: stroke
-    })
-  ];
-
-  // Now that we've run it the first time, replace the function with
-  // a constant version. We don't use an immediately-invoked function
   // and a closure so we don't get an error at script evaluation time in
   // browsers that do not support Canvas. (ol.style.Circle does
   // canvas.getContext('2d') at construction time, which will cause an.error
   // in such browsers.)
-
+  if (goog.isNull(ol.style.defaultStyle_)) {
-  /**
+    var fill = new ol.style.Fill({
-   * @param {ol.Feature} feature Feature.
+      color: 'rgba(255,255,255,0.4)'
-   * @param {number} resolution Resolution.
+    });
-   * @return {Array.<ol.style.Style>} Style.
+    var stroke = new ol.style.Stroke({
-   */
+      color: '#3399CC',
-  ol.style.defaultStyleFunction = function(feature, resolution) {
+      width: 1.25
-    return styles;
+    });
-  };
+    ol.style.defaultStyle_ = [
-
+      new ol.style.Style({
-  return styles;
+        image: new ol.style.Circle({
+          fill: fill,
+          stroke: stroke,
+          radius: 5
+        }),
+        fill: fill,
+        stroke: stroke
+      })
+    ];
+  }
+  return ol.style.defaultStyle_;
 };