When deleting a CelRef, and there are no other Refs to the Cel, the Cel still lingers in the XML
The real issue here is that it starts out as a PNGCel, but after be deleted it destroys the .png file, but leaves in in the XML, so upon the next load/save in the XML it's marked as a 1x1 PNG. Since I haven't implemented complex Cel Library reuse, when A CelRef is deleted and it's Cel has no other Refs, it should also remove the Cel from the CelLibrary.