change fingerprinting algorithm

src/geo/bounds.js

@@ -58,6 +58,17 @@
             UTIL.isCloseTo(this.maxy, bounds.maxy, margin);
     };
 
+    /**
+     * @returns {Number} absolute delta in x,y coordinate space
+     */
+    PRO.delta = function(bounds) {
+        return
+            Math.abs(this.minx - bounds.minx) +
+            Math.abs(this.miny - bounds.miny) +
+            Math.abs(this.maxx - bounds.maxx) +
+            Math.abs(this.maxy - bounds.maxy);
+    };
+
     /**
      * @param {Bounds} b
      */