Fix 16-bit binary PBM sample byte order - #3164
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The binary PGM and PPM formats store 16-bit samples most significant byte first. The decoder and encoder read and wrote them in native little-endian order, so every wide sample was byte swapped. This corrects the swizzled colors reported for 16-bit PPM images. - Reverse the sample byte order in BinaryDecoder and BinaryEncoder. - Fix integer division in the max pixel value upscale factor. - Add exact-value, reference-decoder, upscale, and round-trip tests. - Add a 16-bit PPM test image and its Magick-anchored reference PNG. - Document the binary decoder and encoder in full.
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The binary PGM and PPM formats store 16-bit samples most significant byte first. The decoder and encoder read and wrote them in native little-endian order, so every wide sample was byte swapped. This corrects the swizzled colors reported for 16-bit PPM images.