Blender Git Loki

Geometry Nodes: do not reference custom data layers when copying mesh

We can't use that for now, because the original mesh might be freed
before the new copied mesh. When the original mesh is freed, the shared
layers will be freed as well.

This can probably be improved in the future, so that we can actually
share attribute arrays between meshes. An approach similar to how
geometries and components are shared should work as well.