Before 4369627e71, the Mirror modifier merge did not interpolate
customdata.
Previously, the customdata was simply copied from the original vertex
during the merge, resulting in the retention of flags such as
`MVERT_SKIN_ROOT`.
This fix could just repeat that behavior for mirror. But these flags
can be useful in other cases, so this commit re-adds them to the
resulting merged vertex.
Although this can generate geometries with multiple roots on the same
island, there are different scenarios in which this can happen.
Therefore, multiple roots on the same island is not necessarily
considered a bug.
f8617fe71b