Fix bug during attribute attachment. Cluster labels positions weren't being
set if the root graph doesn't have a label.
Add additional flag for array packing, in which the graphs are
sorted at all but just inserted in input order.
Fix problem with global attribute values. These should have non-empty
values iff every graph defines the attribute and all the values agree;
initialize GD_has_labels for the clone graph as the "or" of this value
for all input graphs.