Changes:
2005/10/31 - replace double backslash by single one in text.
2005/11/14 - generate correct header for Ipe 6.0 preview 25.
2007/08/19 - Alexander Buerger acfb@users.sf.net
             + include some images with anytopnm
             + correct FIG3.1 problem (bugzilla #237)
             + fixed rotation of text and ellipses
             + do not write invisible ellipses / polylines / arcs
             + accept double values in some places (points, thickness, ...;
               extends FIG format)
             + skip comments between FIG objects
             + replaced some %g formats as they illegally appear in the PDF
2007/09/26 - Alexander Buerger acfb@users.sf.net
             + compress bitmap images
             + include compressed JPEG images
2008/04/26 - Alexander Buerger acfb@users.sf.net
             + support grayscale pnm
             + added -g and -p options
2009/10/22 - Alexander Buerger acfb@users.sf.net
             + added -c option to use cropbox for output figure
             + check fgets/fscanf return values
2009/12/05 - Alexander Buerger acfb@users.sf.net
             + write ipe 7 format by default
             + added -6 option to write in ipe 6 format
             + added simple_getopt function to parse options
             + reduce filesize for images by putting 36 bytes per line