Description:
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I create a truecolor image, fill it with 1 color, then I make that color transparent, but when I convert the image to palette, I get 1 pixel of a slightest different color of the first one, shifted 1pixel to the right of every pixel near a transparent pixel.

[GD Version] => bundled (2.0.28 compatible)

Reproduce code:
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<?php
$image = imagecreatetruecolor(100,100) or die("");
$color = imagecolorallocate($image, 255, 229, 178);
imagefill($image, 0, 0, $color);
imagecolortransparent( $image, $color);
imagetruecolortopalette ( $image, 0, 255);
imagepng($image);
?>

Expected result:
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I expect to have all the image transparent.

Actual result:
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With a more complex image, it's as I had moved a selection of the NON transparent image, full of a slightest different color of the one I made transparent, moved 1pixel to the right in the background.