Description:
------------
When copying a single-color transparency true-color image onto a
palette image with imagecopy(), the transparency is lost, if
compiled against the bundled libgd. With external libgd it works
fine.

That bug affects all currently supported PHP versions.

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

$base64 = 'iVBORw0KGgoAAAANSUhEUgAAADIAAAAyCAIAAACRXR/mAAAABnRSTlMAAAAAAABu'
    . 'pgeRAAAAVklEQVRYw+3UQQqAMBAEwf3/p9eTBxEPiWAmWMU8oGFJqgAAuOpzWTX3'
    . 'xQUti+uRJTZ9V5aY1bOTFZLV7yZr9zt6ibv/qPXfrMpsGipbIy7oqQ8AYJED1plD'
    . 'y5PCu2sAAAAASUVORK5CYII=';
$src = imagecreatefromstring(base64_decode($base64));

$dst = imagecreate(50, 50);
$transparent = imagecolorallocatealpha($dst, 255, 255, 255, 127);
imagealphablending($dst, false);
imagesavealpha($dst, true);

imagecopy($dst, $src, 0,0, 0,0, 50,50);

imagepng($dst, __DIR__ . '/test.png');


Expected result:
----------------
An image with red circle on a _transparent_ background.

Actual result:
--------------
An image with red circle on a _black_ background.