Blender Git Loki

USD Exporter: properly export objects whose parents are not exported

The exporter constructs an export hierarchy, and then traverses that
hierarchy from the root to the children. When an object is to be exported,
but its parent is not, it meant that this traversal from (great)parent to
children would skip these objects. This commit fixes that by inserting the
missing parents as 'transform only' into the hierarchy.