Nicer Skies

Sadly, this version again changes the nebulas compared to the last one because of a breaking change with the generation, but this is for compatibility reasons.

Changes:

• Improved Nebula resolution and detail, for a crispier look.
• Compatibility with Custom Stars when twinkling stars are turned off.
• Tweaked settings screen for better consistency

Performance:

• Simplified Nebula algorithm to be slightly faster.
• Only generates parts of the sky that are enabled.