Blender Git Loki

Cycles: Implement refit_nodes for QBVH

Title says it all, quite straightforward implementation.

Would only mention that there's a bit of code duplication around packing node
into pack.nodes. Trying to de-duplicate it ends up in quite hairy code (like
functions with loads of arguments some of which could be NULL in certain
circumstances etc..). Leaving solving this duplication for later.