Description:
------------
See <https://github.com/libgd/libgd/issues/585>.

Test script:
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<?php
$src = imagecreatetruecolor(100, 100);
imagefilledrectangle($src, 0, 0, 99, 99, 0xffffff);
imageline($src, 10, 10, 90, 90, 0x000000);
imagesetinterpolation($src, IMG_BSPLINE);
$dst1 = imagerotate($src, 80, 0xffffff);
imagepng($dst1, 'rot_exp.png');
imageaffine($src, [1, 1, 1, 1, 1, 1]);
$dst2 = imagerotate($src, 80, 0xffffff);
imagepng($dst2, 'rot_act.png');


Expected result:
----------------
both images are identical

Actual result:
--------------
both images are similar, but not identical