Added Horizon Culling for planets.

Added Horizon Culling for planet patches.

This can be extended in future to also entire planets/moons/ships/station etc that are fulling hidden behind planets, or indeed anything spherical :)

Added a couple of new classes and the culling code is based on [Ysaneya's original](http://www.gamedev.net/blog/73/entry-1666972-horizon-culling/).