Description:
------------
Forwarding from upstream[1], since our bundled libgd is affected
as well.

The test script creates an image with four unicolored quadrants.
If the same color is used for the top left and bottom left
quadrants, imagecropauto(…, GD_CROP_SIDES) works as expected.  If
the same color is used for the top right and bottom right
quadrants, imagecropauto(…, GD_CROP_SIDES) fails to crop, though.

[1] <https://github.com/libgd/libgd/issues/486>

Test script:
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<?php

$orig = imagecreatetruecolor(8, 8);
$red = imagecolorallocate($orig, 255, 0, 0);
$green = imagecolorallocate($orig, 0, 255, 0);
$blue = imagecolorallocate($orig, 0, 0, 255);

imagefilledrectangle($orig, 0, 0, 3, 3, $green); // tl
imagefilledrectangle($orig, 4, 0, 7, 3, $red);   // tr
imagefilledrectangle($orig, 0, 4, 3, 7, $green); // bl
imagefilledrectangle($orig, 4, 4, 7, 7, $blue);  // br

$cropped = imagecropauto($orig, IMG_CROP_SIDES);
var_dump(imagesx($cropped));

imagefilledrectangle($orig, 0, 0, 3, 3, $red);   // tl
imagefilledrectangle($orig, 4, 0, 7, 3, $green); // tr
imagefilledrectangle($orig, 0, 4, 3, 7, $blue);  // bl
imagefilledrectangle($orig, 4, 4, 7, 7, $green); // br

$cropped = imagecropauto($orig, IMG_CROP_SIDES);
var_dump(imagesx($cropped));

Expected result:
----------------
int(4)
int(4)

Actual result:
--------------
int(4)
int(8)