im_zoom - zoom an image
       #include <vips/vips.h>
       int im_zoom(in, out, xfac, yfac)
       IMAGE *in, *out; int xfac, yfac;
       im_zoom()  zooms  (ie.  nearest neighbour upsampling) the image held by
       the image descriptor in by factors xfac and yfac and writes the  result
       to  the  image  descriptor  out.   Works  for  any  type of image, even
       LABPACK.
       The function returns 0 on success and -1 on error.
       im_copy(3), im_affine(3).
       National Gallery, 1994.
                                 11 April 1990