Improved ICO and ICNS loading

[radarhere]

Resolves #3875

When calling the ICO load() method, it does not just load the image the first time, but on subsequent times as well, since the size might have changed. Here is an example of the size changing, since this is not normal image behaviour -

>>> from PIL import Image
>>> im = Image.open("Tests/images/python.ico")
>>> im.info['sizes']
{(16, 16), (48, 48), (32, 32)}
>>> im.size
(48, 48)
>>> im.im.size
(48, 48)
>>> im.size = (32, 32)
>>> im.im.size
(48, 48)
>>> im.load()
>>> im.im.size
(32, 32)

This caused the issue in question, as load() is called during save by _ensure_mutable(), and that reloading is overwriting any changes that were made. The solution is to not reload the image if the size is the same.

While investigating this problem, I found that it was not occurring with ICNS. Since ICO and ICNS are similar, I wanted to know what ICNS was doing differently to solve the issue. It turns out that ICNS does not support the size being changed after load like ICO does. So this PR also adds that functionality.