(greymap->colormap greymap)
A greymap is converted to a new colormap with each channel set to the same greymap values.
The resulting new colormap is returned.
> (greymap->colormap (greymap-create 640 480))
#<colormap orow:-1 ocol:-1 xres:-1 yres:-1 w:640 h:480>
>