Feature request: Support positional/ordered dithering

[MobyGamer]

BMP2DHR only supports error-diffusion dithering which is not appropriate for video. Positional/ordered dithering is much more appropriate for video; it is stable across the spatial domain, which translates into being stable across the time domain. Without any changes in the player, this will allow for larger display updates with less dither noise.

Some good free algorithms for doing this are at https://bisqwit.iki.fi/story/howto/dither/jy/