Fix painterSortStable error

Sometimes the result is incorrect.
```js
renderList.forEach(function(o){
  console.log(o.id, o.z);
});
```
![K YP 9G ZWB N D 1 V 5](https://f.cloud.github.com/assets/430824/194967/21757cfc-7fcd-11e2-80a5-8be6e68d279f.jpg)