Description:
------------
The function imagesavealpha() has an opposite effect on the 
output image if the color is marked as transparent. For 
example, the code below should always create a clear image. 
But it renders as black.

The interesting thing is that if you remove the imagesavealpha
, the image renders as expected (clear).

Reproduce code:
---------------
<?php
$img = imagecreatetruecolor(100,100);

$trans = imagecolorresolve($img,0,0,0);
imagecolortransparent($img, $trans);
imagealphablending($img, false);
imagefilledrectangle($img, 0,0, 100,100, $trans);

//Has opposite affect
imagesavealpha($img,true);

header('Content-Type: image/png');
imagepng($img);
?>

Expected result:
----------------
nothing (a clear image)

Actual result:
--------------
a solid black image