Support multiple images

[carbocation]

To create a fixed palette for an animated gif, it would be nice if Quantize could consume multiple images. That way, all images that will go into the gif could feed into the statistics that lead to palette choice. Without that, we either have to (1) pick a "best frame" to feed into the quantizer, or (2) quantize each image separately (which may lead to jumpiness as the color palette shifts between frames).