Blender Git Loki

Nodes: Improve node tree copy performance

When copying a full node tree, we can avoid an O(n^2) loop finding a
unique name for every node if we assume they already have unique names.
That is a reasonable assumption, since unique names are verified
elsewhere when adding a new node.

Copying a node tree with about 4000 nodes took 42 ms before,
now it takes 6 ms.

Differential Revision: https://developer.blender.org/D13644