Sectorview star optimization

[Luomu]

• 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.

[lwho]

Looks a lot better than those flat disks.

One nitpick: The darker parts of the spheres are darker than the hyperjump sphere, which sometimes looks a bit strange. Maybe the spheres shouldn't go to pure black.

[fluffyfreak]

Hah, good timing @robn! I was just reviewing this :D