[Maxima] Draw an animated gif with black background

[Mario Rodriguez]

> How can you send animated gif output to a file, I might as well learn how gnuplot works.  I have been putting it off.  If gnuplot 
> can do animated gifs then that would be more powerful probably.


Draw's animated gifs are made by gnuplot. This is a new terminal
introduced in gnuplot 4.2.

In the first versions of draw, we had to generate all scenes separately
and then make the animation with a third tool (imagemagick, for
example).

Mario