Fix [#37266] Skin modifier can't be copied.

Patch by Martin Felke, many thanks.

When copying that modifier across objects, we also have to ensure that a skin CDLayer is present in dest objects (just as when adding it).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61042 954f8c5b-7b00-dc11-b283-0030488c597c