Description:
------------
Depending on whether GD is built with the bundled or an external
libgd, imagecropauto()'s $threshold argument is treated
differently. External libgd assumes mostly much larger $threshold
values to crop the same amount from the image than the bundled
libgd. The different behavior stems from the different
algorithms[1][2].

Actually, I think the algorithm in the bundled is slightly saner
than the one in external libgd (but both appear to be somewhat
arbitrary). Nonetheless the bundled libgd should be fixed to match
the external libgd; otherwise portable code would have to cater to
these differences.

[1] <https://github.com/libgd/libgd/blob/gd-2.2.3/src/gd_color.c#L14-L16>
[2] <https://github.com/php/php-src/blob/PHP-7.0.12/ext/gd/libgd/gd_crop.c#L350-L352>


Test script:
---------------
<?php

$src = imagecreatetruecolor(255, 255);
$white = imagecolorallocate($src, 255, 255, 255);
imagefilledrectangle($src, 0, 0, 254, 254, $white);

for ($i = 254; $i > 0; $i--) {
    $color = imagecolorallocate($src, $i, $i, $i);
    imagefilledellipse($src, 127, 127, $i, $i, $color);
}

foreach ([0.1, 0.5, 1.0, 10.0] as $threshold) {
    $dst = imagecropauto($src, IMG_CROP_THRESHOLD, $threshold, $white);
    if ($dst !== false) {
        printf("size: %d*%d\n", imagesx($dst), imagesy($dst));
    } else {
        printf("cropped to zero size\n");
    }
}


Expected result:
----------------
[external libgd]

size: 247*247
size: 237*237
size: 229*229
size: 175*175


Actual result:
--------------
[bundled libgd]

size: 253*253
size: 219*219
size: 109*109
cropped to zero size