Add a Node Graph for compose shader

[aiekick]

A node Grpah is a compelxe things, but can be powerfull for those who not love programming.
as an artist tool he can be usefull.

for the moment the node graph is created after parsing of the sahder.

i need to found a way who is the mix bewteen the traditionnal way (shader wrting) and a pure node way ( with all the operation, the glsl offer).

with the possibility for the user to have their own self made function to use in the graph node and created in the glsl file, as part of a library file or not.