possible bug -alpha shape IM 7

[fmw42]

There is a difference in how -alpha shape works between IM 6 and IM 7, but no difference for -alpha copy. In -alpha shape for IM 7, you need to add -negate. I know that IM 7 alpha channels are opposite polarity from IM 6. But I do not think this should be this way as a matter of consistency.

Examples from http://www.imagemagick.org/Usage/masking/#alpha_copy and http://www.imagemagick.org/Usage/masking/#alpha_shape



-alpha copy:

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convert alpha_extract.png -alpha copy alpha_copy6.png

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magick alpha_extract.png -alpha copy alpha_copy7.png

-alpha shape:

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convert alpha_extract.png -background red -alpha shape alpha_shape6.png

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magick alpha_extract.png -background red -alpha shape alpha_shape7a.png

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magick alpha_extract.png -background red -negate -alpha shape alpha_shape7b.png