Volumetric Orion Nebula

Replaces the sprite model used for the Orion Nebula with a volumetric model, the same as procedural nebulae use. Observe that the nebula model will also be added to the pool of models for diffuse procedural nebula. The model is not at all accurate to the three dimensional structure of the Orion Nebula, and the Trapezium cluster is slightly displaced from its center (which gives off light), however it does look reasonably accurate when seen from Earth. In short, not particularly realistic, but quite pretty. It's mostly meant for nice screenshots.