Save additional ICO frames with other bit depths if supplied

[radarhere]

Resolves #6121

Two changes.

1. For im.save("test.ico", sizes=((32, 32), (32, 32)), only save one 32x32 frame, not two.
2. At its simplest, IcoImagePlugin resizes an image to save it at various sizes.

hopper().save("test.ico", bitmap_format="bmp", sizes=[(128, 128), (64), 64)])

At one step more complicated, it allows users to supply the smaller images themselves, through append_images.

hopper().save("test.ico", bitmap_format="bmp", sizes=[(128, 128), (64, 64)], append_images=[hopper().resize((64, 64))])

The user in the issue would like to save the image at different sizes, and also attach additional copies of certain sizes at different bit depths.

hopper().save("test.ico", bitmap_format="bmp", sizes=[(128, 128), (64, 64)], append_images=[hopper().resize((64, 64)), hopper("1")])

This PR allows that.