Issue was indeed in join operation, mesh in which we join all others
could be re-added to final data after others, leading to undesired
re-ordering of CD layers, and existing vertices etc. being shifted away
from their original indices, etc.
All kind of more or less bad and undesired changes, fixed by always
re-inserting destination mesh first.
Also cleaned up a bit that code, it was doing some rather
non-recommanded things (like allocating zero-sized mem, doing own
coocking to remove a data-block from main, etc.).
9f67367f0a