outsourced tile sorting from tile comparing.

src/tiledimage.js

@@ -1931,24 +1931,35 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         }
         previousBest.push(tile);
         if (previousBest.length > maxNTiles) {
-            previousBest.sort(function (a, b) {
+            this._sortTiles(previousBest);
-                if (a === null) {
-                    return 1;
-                }
-                if (b === null) {
-                    return -1;
-                }
-                if (a.visibility === b.visibility) {
-                    return (a.squaredDistance - b.squaredDistance);
-                } else {
-                    return (a.visibility - b.visibility);
-                }
-            });
             previousBest.pop();
         }
         return previousBest;
     },
 
+    /**
+     * @private
+     * @inner
+     * Sorts tiles in an array according to distance and visibility.
+     *
+     * @param {OpenSeadragon.Tile[]} tiles The tiles.
+     */
+    _sortTiles: function( tiles ) {
+        tiles.sort(function (a, b) {
+            if (a === null) {
+                return 1;
+            }
+            if (b === null) {
+                return -1;
+            }
+            if (a.visibility === b.visibility) {
+                return (a.squaredDistance - b.squaredDistance);
+            } else {
+                return (a.visibility - b.visibility);
+            }
+        });
+    },
+
     /**
      * @private
      * @inner