Add atmosphere background shader from scenery

[smlpt]

This feature is already implemented on a scenery feature branch and will be merged soon. Having an alternative to a completely black or uniform box background can be beneficial for longer VR sessions.
The sun position automatically syncs with the local time and can optionally be changed by pressing Ctrl + arrow keys, or Ctrl+Shift + arrow keys. To my knowledge, these keybinds are currently unused in sciview.

• add "current sun time" input field to the inspector
• add "current latitude" input field to the inspector