[Suggestion] Better posterize filter

[sd_snatcher]

Hello,

The current posterize filter is a bit awkward. It only allows the user to select a certain number of colors, similar to what the "change color depth" filter does.

A better color posterization filter would do the same that the GIMP posterization filter does: it allows the user to select how many levels the each color channel can have. Or at least it should let the user select how many bits per color will be used.