It is possible to group objects from different layers, so it is an error
to rely on layer ID when checking visibility of a group. This problem
was partially obscured by the fact that SdrObjGroup::GetLayer() returns
0 if its subobjects are on different layers and 0 is a valid layer ID.

Change-Id: I0ef75544a26817154642185864cafd4e6734fa38
(cherry picked from commit 09d4525b)
Reviewed-on: https://gerrit.libreoffice.org/2962Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>