Node group inputs should behave like cycles now. ---- We create dummy nodes that contains the default values for the nodegroup inputs and link them to the sockets. This way the uniform buffer gathering function can read them. But we also need to disconnect all the internal links to sockets that have hidden values. Theses sockets must not get the values from the nodegroup input sockets if there is no link to them otherwise we get broken results (i.e.: normals for a bsdf node).