#84 Unused palette entries not removed

[Viatrix]

I have an image with 3 colours (all opaque) but the palette contains extra unused entries. I would expect the unused entries to be discarded when optimizing the image with optipng. However, the unused entries are not discarded.

** Processing: pal.png
455x343 pixels, 4 bits/pixel, 5 colors (1 transparent) in palette
Reducing image to 4 bits/pixel, 5 colors in palette