Blender Git Loki

Add initial normal node

In contrast with the read-only "normal" attribute on the face domain,
this node gives normalized values for every mesh domain, including
edges and corners. This is preferrable since it's much more predictable
to work with.

The implementation is a little more complicated than I would like,
but could be improved with const access to lazy calculation of normals
on meshes, which is something I've been planning to work on anyway.