[Bf-codereview] Potential fix for bug #31880 (issue 6332047)

[NicholasBishop at gmail.com]

Reviewers: bf-codereview_blender.org, sergey.vfx,

Description:
Log:

     Fixes for modifier data in multi-user meshes.

     When removing a skin or multires modifier, it skips deletion of the
     associated CustomData layer if the object has any other modifiers of
     that type. This check has been extended to all objects that use the
     object's data.

     Similarly, deleting higher multires levels and multires subdivision
     will not update the maximum level of any other multires modifiers on
     objects that link to the same mesh.

     Note that modifier_apply_obdata() doesn't need any changes as it
     does not allow applying to multi-user data.

     Fixes bug [#31880] instance multiresolution modifier error.

http://projects.blender.org/tracker/index.php?func=detail&aid=31880&group_id=9&atid=498


Please review this at http://codereview.appspot.com/6332047/

Affected files:
   source/blender/blenkernel/BKE_multires.h
   source/blender/blenkernel/intern/multires.c
   source/blender/editors/object/object_modifier.c