Blender Git Loki

Nodes: fix threading issues with node ui storage

Calling BKE_nodetree_attribute_hint_add from multiple threads still
was not safe before..
One issue was that context_map embedded its values directly. So
when context_map grows, all NodeUIStorage would move as well.
I could patch around that by using std::unique_ptr in a few places,
but that just becomes too complex for now.
Instead I simplified the locking a bit by adding just locking a mutex
in NodeTreeUIStorage all the time while an attribute hint is added.

Differential Revision: https://developer.blender.org/D11399