outsourced tile sorting from tile comparing.

src/tiledimage.js

@@ -1931,7 +1931,21 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         }
         previousBest.push(tile);
         if (previousBest.length > maxNTiles) {
-            previousBest.sort(function (a, b) {
+            this._sortTiles(previousBest);
+            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;
             }
@@ -1944,9 +1958,6 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
                 return (a.visibility - b.visibility);
             }
         });
-            previousBest.pop();
-        }
-        return previousBest;
     },
 
     /**