Sectorview star optimization

- Instead of drawing each star with a separate drawcall, collect them into a batch and draw everything at once
- Instead of Drawables::Disk, use a billboard sphere impostor. Perfect little spheres from any distance, with only 4 vertices each.
- Add a bit of shading to each sphere. Someone might later want to tweak the light direction to come from the player, the galactic core or something.

![screenshot-20140202-155133](https://f.cloud.github.com/assets/138378/2061365/2e97fd04-8c4e-11e3-9f8e-a718633586b8.png)