Background node loading in VRMLLoader (no textures yet, colors only)

This loads the Background node and interpolates the skyColor and groundColor according to the skyAngle and groundAngle.

Currently, the sky and the ground are interpreted following the VRML 97 specs as close as possible, with the exception that for now, the size of the ground and sky spheres approximate the size of the earth and its atmosphere. So it is possible to zoom out of atmosphere pretty fast, while according to the VRML 97 spec the spheres should be of infinite size.

No support yet for background images.