Handle variable duration GIF backgrounds

For example, https://media.giphy.com/media/4560Nv2656Gv0Lvp9F/giphy.gif has a slow frame at the beginning that gets lost during conversion.

`duration` can be a list: https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#saving-sequences

Reproduce the GIF conversion with the linked Giphy image, then trace where Pillow saves the animated background and how the duration value is passed. Use Pillow's sequence-saving documentation as the starting reference; done means the slow opening frame is retained during conversion.

Written by the indexing model from the issue text.