Fix SkatterSky flare occlusion.

Fixes #764 - ScatterSky Flare Rendering - Occlusion Is Broken
Use this [terrain for easy check](https://www.dropbox.com/s/q2iojkq1etgfggt/Empty%20Terrain.mis?dl=0)

Before:
![screenshot 2014-12-08 23 01 08](https://cloud.githubusercontent.com/assets/2388307/5348761/c3dcaeb4-7f32-11e4-95a3-490482f0adbb.png)

After:
![screenshot 2014-12-08 23 02 46](https://cloud.githubusercontent.com/assets/2388307/5348764/c58ce846-7f32-11e4-8474-3e4444d2c9e7.png)