(greymap-scale-absolute greymap new-width new-height)
Passing 0 in either new-width or new-height leaves the aspect ratio intact.
A new scaled greymap is returned with the desired width and height.
> (define gm (greymap-create 1024 768))
gm
> (greymap-scale-absolute gm 640 480)
#<greymap orow:-1 ocol:-1 xres:-1 yres:-1 h:640 w:480>
>