Overlay images that have pure black values flicker white

[francoislg]

When any transparency is set, whichever is the value, gifencoder will try to get the "closest" color to it from the gif bitmap. It sets "Pure black" as the transparent color. This results in overlay images that flicker white when they are only black and white.

Also happen with the (soon) new base parrot when the lines were pure black. I "fixed" it by replacing by hand "rgb(0,0,0)" to "rgb(1,1,1)"...

Somewhat related to this fix, since it is needed to make gifencoder work with transparency: eugeneware/gifencoder#5