Really fix polar-to-cube

! Now tested (direction change was missing; e.g. on the left cubemap
  face, when advancing horizontally, you want to go towards negative Z,
  whereas on the right cubemap face, you want to go towards positive Z)
Add GraphicsStatus::aspect, use vec4::position()

+ GraphicsStatus::aspect makes things easier
+ vec4::position() exists, use it
Change star map brightness function

* Make it more flat (the new star map looks better, so this is fine)
Moon

+ Add moon