Fix #37084, Backdrop not invalidating inside node groups.
Extended the is_active_group flag such that both the current edittree as well as the base node tree in Scene do a viewer node update.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60798 954f8c5b-7b00-dc11-b283-0030488c597c