Fix visual artifacts in Blurry Halo Selection

18953da958bc292c0a2 introduced a depth buffer to the FBO with the apparent intention to show outlines for units behind a <i>unit that already has outline</i> (?). The code never called `gl.DepthMask(true)` so nothing was ever written to the depth buffer and it ended up containing garbage.

I disabled depth testing and removed the depth buffer. This doesn't change the way selection is rendered and fixes the bug.

## Screenshots illustrating the bug

![screen00321](https://cloud.githubusercontent.com/assets/2241548/4692987/aefadf28-5784-11e4-8808-a6c0e056dc2e.jpg)
![screen00322](https://cloud.githubusercontent.com/assets/2241548/4692988/aefb05a2-5784-11e4-95d6-18a65ba6f2aa.jpg)
![screen00323](https://cloud.githubusercontent.com/assets/2241548/4692986/aef98484-5784-11e4-812e-91a606790fc5.jpg)