Node tree updates can crash if the tree contains a node group that points at an "undefined" tree type.
This can happen if the tree is linked from a library and the path is lost,
or if a custom (python) tree is used and the script is not run.
The fix is to check if the node group type is valid ("registered") and return an empty list otherwise.
Pull Request: blender/blender#105564
Previously, the lifetimes of anonymous attributes were determined by
reference counts which were non-deterministic when multiple threads
are used. Now the lifetimes of anonymous attributes are handled
more explicitly and deterministically. This is a prerequisite for any kind
of caching, because caching the output of nodes that do things
non-deterministically and have "invisible inputs" (reference counts)
doesn't really work.
For more details for how deterministic lifetimes are achieved, see D16858.
No functional changes are expected. Small performance changes are expected
as well (within few percent, anything larger regressions should be reported as
bugs).
Differential Revision: https://developer.blender.org/D16858
