Support material export for duplicated objects

[harag-on-steam]

Currently objects will be missing materials on export if they partly consist of other duplicated objects and those objects are not selected by the export graph.

This would require to check the dupli settings of an exported object and then to recursively gather materials from the duplicated objects. The most straightforward to gather should be dupli-groups.

How does Blender's .fbx exporter handle this?