Problem: if a submodel has a subsubmodel, then its image as a callout may not include the subsubmodel.
Reason: when the submodel is rendered, the renderer does not know about the subsubmodel since its
temporary file has not been written to the disk yet.
Solution: when a submodel is complete, write the corresponding temporary file to the disk right away.
File bug4.mpd is an example showing the bug
File bug4_patch.txt is a patch correcting the bug