Order of colors in PLTE not kept anymore

[petrdiblik]

At some point between 2015 and 2020, ZopfliPNG changed the palette processing from "keep the order of colors in PLTE" to "reorder the palette as the colors are found in the image".
But in many cases, the former approach gives better results.
Before any more sophisticated algorithm is added, would it be possible to allow keeping the PLTE order? It could be done by "--keepchunks=PLTE" (this currently doesn't work as desired) or by adding a new parameter, e.g. "--keep_palette_order".
Of course there would be some consequences to consider (e.g. the input and/or the output aren't indexed, the input PLTE contains unused colors).