Fixes to 3D clouds

Fixes to 3D clouds
 - Wrap 3D clouds (e.g. Global Weather) on the surface of a sphere rather than on a plane. Cloud altitude no-longer increases with distance from the start position
 - Fix altitude of individual clouds, which where being places approx 1/2 the sprite height too high. Note that this means sprite positions in the cloud are the center of the cloud rather than the bottom.
 - Improve LOD radii, which were far too large.