Please port OrderedPosterizeImage to IM7

[glennrp]

The OrderedPosterizeImageChannel method has not been ported from IM6 to IM7, so IM7 falls back on the original bi-level OrderedDitherImage method.

Given a black-to-white gradient image, compare IM-7.0.2-9

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magick gradient.png -ordered-dither o8x8,6 od_o8x8_6_im7.png

which has only black or white pixels with IM-6.9.5-7

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convert gradient.png -ordered-dither o8x8,6 od_o8x8_6_im6.png

whch has six levels of gray pixels