Description:
------------
When merging two images, one normal and the other one completely black, the output from imagecopymergegray produces a weird looking mosaic of the first.

Reproduce code:
---------------
<?php
$imPng = imagecreatefrompng("Tiles/Bamboo/9.png");
$imSrc = imagecreatefrompng("Tiles/Bamboo/black.png");
$height = imagesy($imPng); 
imagecopymergegray($imPng, $imSrc, 0, 0, 0, 0, $height, $height, 50);
header("Content-type: image/png");
imagePng($imPng);
?>
See also 
http://www.tilehog.net/prepermission/showbug.html

Expected result:
----------------
The original image should be darkened by 50%. This was the result in the earlier version (4.3.10).

Actual result:
--------------
http://www.tilehog.net/prepermission/showbug.html