ppm3d - convert two portable pixmap into a red/blue 3d glasses pixmap
       ppm3d leftppmfile rightppmfile [horizontal_offset]
       Reads  two  portable  pixmaps  as input.  Produces a portable pixmap as
       output, with the images overlapping by horizontal_offset
       pixels in blue/red format.
       horizontal_offset defaults to 30 pixels.   Pixmaps  MUST  be  the  same
       size.
       ppm(5)
       Copyright (C) 1993 by David K. Drum.
                                2 November 1993                       ppm3d(1)