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
b63e9a96df