Consider removal of the `Skymap (spherical)` sky mode

[Peregrine05]

As nothing is known about the type of image to be used in this format, and such images do not seem to exist anywhere, this option shoud be considered for removal, as it might also cause confusion to new users.

[jackjt8]

More importantly any projection that converts between a sphere and a plane is regarded as spherical in nature. Meaning that when you search for spherical skymaps you typically find equirectangular skymaps. Researching spherical projection also returns a multitude of results; at least 70 of them within one paper I found. While it may be possible to invert the process of the Spherical skymap case to produce a debug image, matching this image up to a particular projection mode will take additional time and effort for a feature which is likely not going to be used.

I am for removing this Skymap mode.

[Peregrine05]

The Skymap (spherical) sky mode is actually meant to be used with "light probe" skymaps, which are in "angular fisheye" projection (http://www.paulbourke.net/dome/fisheye/).

Comment in Sky.java confirming this: https://github.com/chunky-dev/chunky/blob/668a3a27f5dc85c3b019189cb85d53754ce452bf/chunky/src/java/se/llbit/chunky/renderer/scene/Sky.java#L112