Check validity of selected 3D vector objects, and display an error message
if one of the selected images is not a valid 3D vector object.
Full 3D object check is slower but more precise.
G'MIC is an open-source software distributed under the
CeCILL free software licenses (LGPL-like and/or
Copyrights (C) Since July 2008,
David Tschumperlé - GREYC UMR CNRS 6072, Image Team.