Updated rendering of cold stars
Upgrades in cool star rendering. Stars are red dwarfs M8.1, M9.0, M9.9. Brown dwarfs are L6, T5, Y5. Note what late stars have almost the same size. And yes, SpaceEngine now supports decimal subclass numbers like "8.1". Solar spots are upgraded a bit also. There must be a smooth transition in appearance between coldest M-dwarfs (M9.9) and hottest Y-dwarfs, but I didn't implement it yet.