Cosmetic fixes for different projection schemes.
The opaque ground polygon is now well-behaved for all projections. Fixed a crash at low zoom when using Gnomonic projection, as reported by Mederic Boquien.

Gnomonic is a special case, because unlike the other projections, it has an infinite "sky horizon" boundary. i.e., the locus of points that are 90 degrees from the focus is projeced as a circle in other systems, but these points are at infinity in the Gnominic projection.

Also forward-porting the fix for Bug #133505 (crash condition in SkyMap::refract()). Will backport these changes to the 4.0 branch as well.