Prismarine Shader

1) Reworked nebulas! Now they appear in form of large distorted spikes which sometimes take almost 100% of the skybox. Are very dynamic and can have various shapes and sizes.
2) Modified fog. Added sliders for morning/day/evening/night/weather density.
3) Tweaked caustics
4) Reworked volumetric fog! Now it has a form of a layer of low-altitude volumetric clouds in the Overworld. Underwater and End fog are still the same.
5) Modified sky. Slightly changed its colors
6) Improved performance a little bit