im_falsecolour - colour a mono image
       #include <vips/vips.h>
       int im_falsecolour(in, out)
       IMAGE *in, *out;
       Turns  a  one-band  unsigned char image into a three band unsigned char
       image.  The colour scale is stolen from a PET scan and has red for hot,
       green for normal and blue for cold.
       0 on success and -1 on error.
       im_disp2XYZ(3), etc.
       National Gallery
       J. Cupitt
                                30 October 1992