extend colormap in indexed pictures

[Ofnuts]

I don't know how to do it in script-fu, but as far as I remember, you get the color map as a flat list of bytes which is 3x the number of colors and where RGB are three consecutive items in the list. So if you want to extend the map with black you have to add (0,0,0) at the end.

See invert-colormap-0.0.py for Python sample code.