Description:
------------
On platforms where `char` is actually `signed char` an integer
overflow can happen in `gdImageScaleBilinearPalette` due to sign
extension when `red`, `green` and `blue` are passed to
`gdTrueColorAlpha()`.

This issue has been reported against upstream libgd[1], but as
PHP's bundled libgd is affected as well, I'm forwarding.

[1] <https://github.com/libgd/libgd/issues/330>


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

$src = imagecreate(100, 100);
imagecolorallocate($src, 255, 255, 255);
$dst = imagescale($src, 200, 200, IMG_BILINEAR_FIXED);
printf("color: %x\n", imagecolorat($dst, 99, 99));


Expected result:
----------------
color: ffffff

Actual result:
--------------
color: fffffffffffefeff