Support saving uncompressed ICO

[pdknsk]

Currently Pillow basically writes multiple PNGs into an ICO container, which is not supported by some applications that can read ICO. I'm not familiar with the ICO format, but I think biCompression here references the possible sub-formats.

[aclark4life]

❓ Is this a feature request?

[aclark4life]

Duplicate of #2513?

[pdknsk]

It's a different issue.

[JamesTheAwesomeDude]

Duplicate of #2513?

@pdknsk That issue was about sub-images; this issue is not a duplicate. This is a feature-request (which I share) for non-compressed ICO files; could it be re-opened?

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The following is hard-coded:

When saving, PNG compression is used. Support for this was only added in
Windows Vista.

And, unfortunately, this is triggering mate-desktop/eom#310
(I'm not saying that that software isn't at fault, just that this issue is triggering that one)

[radarhere]

So from https://docs.microsoft.com/en-us/previous-versions//dd183376(v=vs.85)?redirectedfrom=MSDN, you're after "BI_RGB", "An uncompressed format", yes? Looking at https://en.wikipedia.org/wiki/ICO_(file_format), I'm guessing this would be BMP, instead of PNG. Does that sound right?

[radarhere]

I've concluded that the BITMAPINFOHEADER structure isn't used, so BI_RGB is irrelevant.

I've created #5513 to add BMP format, through im.save("out.ico", bitmap_format="bmp"). Let us know if you have any thoughts.