Improve code for setup of global_func_set in ShaderLanguage

[Chaosus]

That how #93849 should be, I hurried

[akien-mga]

I wonder about thread safety, is that fine? CC @RandomShaper

I assumed ShaderLanguage didn't have multiple instances, but I guess it does?

[Chaosus]

@akien-mga There are multiple shader compilers one for each type of shader, each of them contain its own ShaderLanguage instance - so it's more logically to add some kind of code safety, at least to not repeat the same process of filling the same array... dunno about the aspect of multithreading

[akien-mga]

Thanks!