Description:
------------
If the function imagecreatetruecolor function is used instead of imagecreate the transparent color seems to get lost when scaling down the color palette if the image is exported as gif.

Reproduce code:
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<?php
header ("Content-type: image/png");
$im = @imagecreatetruecolor(50, 100)
      or die ("Kann keinen neuen GD-Bild-Stream erzeugen");
#$im = @imagecreate(50, 100)
#      or die ("Kann keinen neuen GD-Bild-Stream erzeugen");
$background_color = ImageColorAllocate ($im, 100, 100, 100);
$rect_color = ImageColorAllocate ($im, 200, 0, 0);
ImageFilledRectangle($im,0,0,50,100,$backgroundcolor);
ImageFilledRectangle($im,0,0,50,50,$rect_color);
$text_color = ImageColorAllocate ($im, 233, 14, 91);

ImageString ($im, 1, 5, 5, "Ein Test-String", $text_color);
$res = imagecolortransparent($im, $background_color);
#var_dump($res);var_dump($background_color);
ImageGIF($im);

?>