Blender Git Loki

Fix for bug 32017.

There was some bad recursion introduced recently that caused crashes
when a Material node is the same material as the material itself (e.g.,
if Material.001 has a node with Material.001).

This commit attempt to correct this by keeping track of the material
at the root of the node tree, and doesn't recurse further if it
encounters it again within the nodetree.

Joshua, please review!