Image viewer incorrectly previews Windows ico as garbage pixels if last texture size is 256

[Long Nguyen Huu]

ImageViewer version: 42.0

ImageMagick version (should not matter, ImageMagick seems to be doing its work correctly and it's in another package, but dropping just FYI):
$ convert --version
Version: ImageMagick 6.9.11-60 Q16 x86_64 2021-01-25 https://imagemagick.org
Copyright: (C) 1999-2021 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC Modules OpenMP(4.5)
Delegates (built-in): bzlib djvu fftw fontconfig freetype heic jbig jng jp2 jpeg lcms lqr ltdl lzma openexr pangocairo png tiff webp wmf x xml zlib

I was following instructions on https://askubuntu.com/questions/867567/convert-jpg-or-png-to-ico-using-terminal-and-back and https://superuser.com/questions/491180/how-do-i-embed-multiple-sizes-in-an-ico-file to generate a Windows icon from separate pngs.

While instructions like:

$ convert icon_256.png -define icon:auto-resize=256,128,64,48,32,16 icon.ico
and
$ convert icon_256.png icon_128.png icon_64.png icon_32.png icon_16.png icon.ico

will generate a Windows .ico that can be previewed without issues in Image Viewer, although it will only show the 128x128 image, not the bigger 256x256 (not sure if it's a bug or by design).

If you swap sizes to put 256 at the end (you can also switch to ascending sizes but it's not necessary):

$ convert icon_256.png -define icon:auto-resize=128,64,48,32,16,256 icon.ico
and
$ convert icon_128.png icon_64.png icon_32.png icon_16.png icon_256.png icon.ico

will generate a proper .ico (textures are simply stored in a different order, as can be shown by extracting them with convert icon.icon extracted_icon.png) and looking at the generated files in order of numerical suffixes.

but when opening icon.ico, ImageViewer will show a 128x128 image with only the bottom part filled with garbage pixels (see screenshot).