Make shadergen emit function definitions for all uses of nodegraphs

[niklasharrysson]

Currently only the nodegraphs that are node implementations gets treated as separate function definitions. Nodegraphs that are just "compounds" are instead traversed through, extracting their nodes, when constructing the ShaderGraph to generate code from.

If we treated all nodegraphs as seprate function definitions it would simplify code generation. It would also avoid issues with name collisions for nodes inside vs outside a graph, if all nodegraphs resulted in separate ShaderGraph instances.