Smoother Glowing

• 1.21.9 support (wb backwards compat, realized overlays exists)

• 1.21.6 support (bye bye backwards compat)

• Make it compatible with 1.21.5+

• Make it compatible with 1.21.2+

• Optimizes even more, although it's small.
• Refactor the code and add comments for readability.

• Deprecate rcp() since it doesn't work on all cases
• For some reason Iris mod need Sampler2, so I added it just to make it work

Basically if v1.0.0 already works for you then this version is not necessary