Realizing and copying attributes of meshes, curves, and points are very
similar processes, but currently the logic is duplicated three times in
the realize instances code. This commit combines the implementation
for copying generic attributes and creating the result id attribute.
The functions for threaded copying and filling should ideally be in
some file elsewhere, since they're not just useful here. But it's not
clear where they would go yet.
Differential Revision: https://developer.blender.org/D14294
338fe9e2b5