Blender Git Loki

Cycles: Add method to dump current shader graph to the graphiz file

This is rather useful to see how good optimization went and so.

Currently uses quite simple notation: shader nodes are nodes on the
graph, connects between graph nodes are named by the sockets names,
so i.e. connection between BSDF and Mix would be named bsdf:closure1.

Could be improved in the feature to draw fancier graph, but it's good
enough already.

Use in the following way:
- To create graphix file call graph->dump_graph("graph.dot")
- To visualize the grapf call: dot -Tpng graph.dot -o graph.png