An invalid ICO image is generated when it includes multiple BMP images

[sorairolake]

Discussed in #2185

^{Originally posted by sorairolake March 26, 2024}
I wrote a code to create a ICO file containing multiple BMP or PNG images:

https://github.com/sorairolake/favico/blob/1c5cb360350097900988bb407c81368d55512648/src/generate.rs#L40-L59

The generated ICO file containing multiple PNG images seems correct, but it containing multiple BMP images seems corrupted:

$ identify png/favicon.ico
png/favicon.ico[0] PNG 16x16 16x16+0+0 8-bit sRGB 269B 0.010u 0:00.000
png/favicon.ico[1] PNG 32x32 32x32+0+0 8-bit sRGB 323B 0.010u 0:00.000
png/favicon.ico[2] PNG 48x48 48x48+0+0 8-bit sRGB 1027B 0.000u 0:00.000

$ identify bmp/favicon.ico
identify: improper image header `bmp/favicon.ico' @ error/icon.c/ReadICONImage/400.

How can I create a ICO file containing multiple BMP images which is not corrupted?