Description:
------------
Since GIF does not support truecolor images, a palette version of
the image is created with `gdImageCreatePaletteFromTrueColor()`
internally.  When actually converting to GIF, our bundled libgd
uses the interlace flag of the palette image, although it is not
retained by `gdImageCreatePaletteFromTrueColor()`.  Upstream libgd
correctly uses the interlace flag of the original (truecolor)
image instead.

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

$im = imagecreatetruecolor(10, 10);
imagefilledrectangle($im, 0, 0, 9, 9, imagecolorallocate($im, 255, 255, 255));
imageinterlace($im, 1);
imagegif($im, __DIR__ . '/test.gif');

$im = imagecreatefromgif(__DIR__ . '/test.gif');
var_dump(imageinterlace($im));

Expected result:
----------------
int(1)

Actual result:
--------------
int(0)