notcurses sixel quantization now as good as libsixel

[dankamongmen]

with the merging of #2545, my new design for sixel quantization is complete, and we're now just as fast as libsixel on most images, significantly faster than libsixel on images that occupy a large portion of the RGB space, and our quality seems about as good as undithered libsixel.

i'm delighted with these results. i honestly didn't think they would come this easily.

@ctrlcctrlv, we might indeed want to start talking about backing libsixel with notcurses.