Support dithering with tiles #60
Description
Related to a question that came up in the Julia Slack #image-processing channel a while ago, it would be nice to dither using smaller images as tiles.
A simple way to implement this would be to average colors in each tile to obtain a color scheme. We would then obtain an IndirectArray from which it would be possible to reconstruct the tiled image.
One open issue is how to deal with non-square tiles. It might be necessary to rescale the input image to compensate for stretching of the output image.