Compiling this big file for every Eevee material is bad for performance, and
now that we are adding more nodes it gets worse. This patch adds a simple
mechanism to split up that file, and use only the parts used by shader nodes.
When a function is used by GPU_link, we detect which GLSL file it came from
and use it in GLSL code generation automatically.
This does not handle dependencies between GLSL files, so each file must be
self contained and not have conflicting function names.