Description:
------------
When I use imageCreateTrueColor instead of imageCreate, and I call imagecolorAllocate(), the background color won't be changed.
I got a black background instead of a green. My gd version is bundled (2.0.28 compatible)

Reproduce code:
---------------
<?php
$image1=imageCreate(100, 100);
$image2=imageCreateTrueColor(100, 100);
$green1=imageColorAllocate($image1, 0, 128, 256);
$green2=imageColorAllocate($image2, 0, 128, 256);

print'In case of imagCreate(): ';
if(imageColorAt($image1, 1, 1)==$green1){
	print'Ok, background is green.';
}else{
	print'Background is black.';
}

print'<br>In case of imagCreateTrueColor(): ';
if(imageColorAt($image2, 1, 1)==$green2){
	print'Ok, background is green.';
}else{
	print'Background is black.';
}
?>

Expected result:
----------------
In case of imagCreate(): Ok, background is green.
In case of imagCreateTrueColor(): Ok, background is green.

Actual result:
--------------
In case of imagCreate(): Ok, background is green.
In case of imagCreateTrueColor(): Background is black.