Potential fix for bug #31880

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

[sergey.vfx, 2012-06-22 07:24:08]

Just sone more possible issue: when you join objects synchronization of
subdivision level happens in multiresModifier_prepare_join(). This could have
the same issue if you're joining objects with different multires levels and
which have got seevral instances.

Apart from this - LGTM.

[nicholasbishop, 2012-06-22 17:01:12]

On 2012/06/22 07:24:08, sergey.vfx wrote:
> Just sone more possible issue: when you join objects synchronization of
> subdivision level happens in multiresModifier_prepare_join(). This could have
> the same issue if you're joining objects with different multires levels and
> which have got seevral instances.
> 
> Apart from this - LGTM.

Good point about joining. Could be simply fixed by updating the level for other
instances, but I wonder if this is the right thing to do? From the user
perspective, it might seem a bit odd that joining two meshes would modify other
instances even though at the end of the join op the instances aren't linked to
the original input to the join op anymore.

Not sure how to simply fix it otherwise though, so adding patch with just
updating levels.

[nicholasbishop, 2012-06-22 17:01:45]

Fixes for joining multires mesh objects

[sergey.vfx, 2012-06-24 12:30:10]

On 2012/06/22 17:01:45, nicholasbishop wrote:
> Fixes for joining multires mesh objects

Well, i'm not sure how to prevent changes in instanced objects in a nice way
too. This could probably be done via making dupliobject real and then joining,
but that's quite tricky.

I'd say current patch is fine.

[nicholasbishop, 2012-06-24 20:19:17]

On 2012/06/24 12:30:10, sergey.vfx wrote:
> On 2012/06/22 17:01:45, nicholasbishop wrote:
> > Fixes for joining multires mesh objects
> 
> Well, i'm not sure how to prevent changes in instanced objects in a nice way
> too. This could probably be done via making dupliobject real and then joining,
> but that's quite tricky.
> 
> I'd say current patch is fine.

Committed as r48247, thanks for reviewing.