The merging behavior of the Array, Mirror, Screw modifiers has changed
since 4369627e71.
Before, only the customdata of the destination vertex was kept. Source
vertices were completely removed.
With 4369627e71, all vertices in the merge group (whether source or
destination) now have their customdata interpolated.
But this can cause problems for example with the root vertex customdata
of the skin modifier, where if only one of the vertices of the group of
vertices has this flag, the resulting one will also have it.
This commit restores the behavior for the vertices customdata and does
not interpolate it if `mix_data` is false.
Germano Cavalcante