Fixed broken random tiles in map editor for RA snow tileset

Before:

![image](https://cloud.githubusercontent.com/assets/756669/3077122/f6d906b4-e414-11e3-836c-934e3bd4bdde.png)

After:

![image](https://cloud.githubusercontent.com/assets/756669/3077126/1975ed7c-e415-11e3-917c-dfd13ee7c599.png)