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