Backport the ClosestColor improvement from 7.1.0-58 to ImageMagick-6.

[yoya]

Prerequisites

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Description

Backport the ClosestColor improvement from 7.1.0-58 to ImageMagick-6.
Only for alpha calculation and does not reflect Hue support reverting.

• Since ImageMagick-6.9.12-71, the color reduction process for alpha images has been broken until 6 main:HEAD.
• ImageMagick-7.1.0-56 had a similar breakage, which was fixed in ImageMagick-7.1.0-58
• ImageMagick-7 ClosestColor function fix has not been backported to ImageMagick-6.

The difference is noticeable in images where RGB is flat and A is used to represent the vision

test image

% magick -size 640x480 xc:gray  \( logo: -type grayscale \)  \
          -compose CopyOpacity -composite  logo-trans.png

logo-trans.png	(RGB component)	(alpha component)

-colors 2 behavior by version

% convert logo-trans.png -colors 2  colors2.png

... 6.9.12-70	6.9.12-71 ... 6.9.12-72	6.9.12-73 ... 6.9.12-93 main:HEAD
... 7.1.0-55	7.1.0-56 ... 7.1.0-57	7.1.0-58 ... 7.1.1-15 main:HEAD

reference code

• ImageMagick/ImageMagick@f220cb9

remarks

I don't know why Hue support was reverted, so I have only ported the alpha calculation fixes in this commit.
If Hue needs to be addressed as well, someone please make an additional commit.

ImageMagick-7 changes the type of alpha, beta, and distance from MagickRealType to double, but I have not included it in the backporting because of the possibility that it was just caught up in the revert process.

[yoya]

I also backported hue support reverting, which is inseparable from the correction of the alpha calculation for color reduction.