Blender Git Loki

Fix T49423: Data Preview of group containing only group instances is empty.

Code was not getting correct boundbox in some cases (group only instancing other groups e.g.), now
compute our own bbox in those cases.

Based on patch by @lichtwerk, but extended the fix to include linked datablocks in some cases
(linked objects in local groups, linked objects in local scene, etc.), this was also broken in existing code.

Reviewers: mont29

Subscribers: duarteframos

Differential Revision: https://developer.blender.org/D2257