Blender Git Loki

IO: ensure export path and export name are always consistent

Before this, there was one code path that set `context.export_path`, and
a different code path for `context.export_name`, allowing the two to
diverge.

Keeping track of the export path of the export parent (which can be, but
is not always, the Blender parent object) also allows a concrete
subclass of `AbstractHierarchyIterator` to find the `AbstractWriter` for
the export parent. In case of exporting to Alembic this is important, as
it's not possible to simply give the Alembic library the full export
path of an object like we do in the Universal Scene Description (USD)
exporter; Alembic needs the C++ object of the parent.