Start work on a two-pass rendering model for near/far objects

Bugs:
 - internal views are messed up
 - planets go wrong once they leave the near rendering pass
 - horribly unoptimised so far
Fix internal views
More two-pass rendering work

A bit of optimisation, fix some depth-buffer issues.
Improves but doesn't fix planets - still a problem with atmosphere.
Adjust the boundary between the two render passes

Adjusting the boundary to 100m means that most of the rendering takes place with a near clip plane of 100, which gives acceptable results on a 24-bit depth buffer at visual ranges.

There's still a need to do a short-range rendering pass for the few objects in that region, but it's fairly cheap.
Go back to cheating on planet atmosphere rendering

Even at 100m near plane the depth buffer isn't good enough at wide distances
Sort out planet atmosphere renderings
Merge pull request #97 from OoliteProject/two-pass-rendering

Two pass rendering