int im_zoom(in, out, xfac, yfac) IMAGE *in, *out;int xfac, yfac;
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.