when removing, only descend into children that can contain the node t…

…o remove

A node's bbox is the union of its children's bboxes.  When searching for
a node with a specific extent, the node can only be in children who's
bboxes strictly contain the node's extent.  We can therefore replace the
intersection test with a stricter contains test.  This improves the
speed of remove by about 50%.

rbush.js

@@ -147,7 +147,7 @@ rbush.prototype = {
                 }
             }
 
-            if (!goingUp && !node.leaf && this._intersects(bbox, node.bbox)) { // go down
+            if (!goingUp && !node.leaf && this._contains(node.bbox, bbox)) { // go down
                 path.push(node);
                 indexes.push(i);
                 i = 0;