Return a 2D size width,height
which is bigger than min_width,min_height
and has the specified w/h ratio.
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.