Description:
------------
When upscaling a completely white image with the interpolation
method IMG_BILINEAR_FIXED, the resulting image has a black border
at the bottom.

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

[1] <https://github.com/libgd/libgd/issues/329>


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

$src = imagecreatetruecolor(100, 100);
$expected = 0xffffff;
imagefilledrectangle($src, 0,0, 99,99, $expected);
$dst = imagescale($src, 200, 200, IMG_BILINEAR_FIXED);
printf("color: %x", imagecolorat($dst, 199, 199));


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


Actual result:
--------------
color: 5f7f7f7f