Initial 1.21.8 port

[imreallybadatnames]

Quite a lot of changes. This initial version is on par with the 1.21.4 version in terms of functionality.
Also, before testing, please note that it depends on an unpublished version of lavender-md that I published locally on my machine via mavenLocal: 0.1.2+1.21.8 is identical in code to 0.1.2+1.21.5, the only difference being the FAPI dependency version, 0.133.4+1.21.8, which prevents the lavender devenv from loading 1.21.5 FAPI modules and consequently throwing a Mixin application error.
Notes:

• The in-world structure overlay is a a tiny bit wonky with shaders, but from my testing, this is better than the 1.21.4 version, since that version doesn't render at all.
• The book's structure component rendering is also a bit weird, but from what I've seen, it's probably impossible to fix that fully.
• Simplified the blit_alpha fragment shader to have a constant alpha value, since AFAIK that's the only usage of the shader, and also because I couldn't bother fixing the uniform value uploading.
• Also, during porting, I left some code comments with specific questions about the old code. It would be preferable to have them resolved before merging the port.