Indexed pallete support?

[baryluk]

I have pretty big images that are mostly screenshots or screenshot mosaics , and with some minor quality loss using less than 24-bit per pixel does provide substantial space savings (and make compression job a bit easier too). It would be nice to be able to use 6, 7 or 8 bit per pixel with optimized pallete, or 16-bit per pixels using RGB (5+6+5, or 5+5+5).