Add Macintosh filter

This filter implements the dithering used on early Macintosh computers. It doesn't downscale like cga does, since the Macintosh uses a much more organic dithering pattern, making the resolution (at least in my opinion) less important.

Original:

Filtered:

This filter is much faster than cga since it doesn't involve either the somewhat slow multiplies (for cga/cga_unscaled) or the very slow CIEDE2000 comparisons (for cga_hq).