softgpu : Fix depth issue in FF Type-0

@neobrain , not too sure if it is sensible here but anyway it did fix the depth issue in FF Type-0

Before 
![screen00018](https://f.cloud.github.com/assets/3000282/982615/68101a3e-080b-11e3-8ef3-9485157e4833.jpg)

After
![screen00019](https://f.cloud.github.com/assets/3000282/982616/6a7906f0-080b-11e3-938e-d92ede37f036.jpg)