Add separate functions that deal with the vertex domain and copy vertex
groups without using the attribute API which has a large overhead when
abstracting the access of many vertex groups.
In a 1m vertex mesh with 20 vertex groups, I observed an improvement
in the node's runtime from 399 ms to 64 ms.
Also resolves#117553. That was an error when adding weight data to a
mesh without any weight data would invalidate custom data layers. That
is solved more simply now by just doing nothing in that case.
6aaa74cda9