Things which have not yet been done for this release but which I will
hopefully have done before one of the next releases:

  - routine which separates greyscale/color images into bit planes and joins
    bit planes again to greyscale/color images

  - pgmtojbg and jbgtopgm

  - routine which prepares incompletely decoded images for Xlib

  - call-back function which allows to implement a graphical progress meter
