Implement optifine's shader API

[pevernow]

At present, there are a lot of signs that among the developers of minetest, there are few who are proficient in advanced shaders.

Minecraft's optifine is a good shader loading plug-in.

If minetest can support loading some originally optifine based shaders in the same way, it may make minetest look more beautiful, and users can freely choose shaders.

There is no need for developers to write shaders