Instead of modifying the active mesh in place, which means we can't
use the size of its data arrays when copying its data, and its caches
are immediately invalidated, copy data to a separate out-of-main
result mesh first. The only downside is that for a moment during
the operator the shape key array sizes will be out of sync with the
mesh size.
Also the custom data for multires layers wasn't copied properly
after the recent refactor that rewrote this code. Take the opportunity
to fix that too.
The motivation for this change is an improvement to copy different
kinds of custom normals properly to the joined mesh, which never
worked since free custom normals were introduced.
This contains a few changes to the expected results in the tests.
Those are edge cases, and the new results make more sense.
