remove offset scale from matrix

Slot's matrix.tx/ty is mutipled by "this._parent._global.scaleX":
![image](https://cloud.githubusercontent.com/assets/2263255/4843065/19ad5c1e-602d-11e4-844a-b5d7d4f31c24.png)

The "_parent._global.scaleX" is already mutipled by "_offset.scaleX":
![image](https://cloud.githubusercontent.com/assets/2263255/4843030/d9bb5458-602c-11e4-9cfc-0949522f63c2.png)