[WIP] uploads: Add support for resizing GIF with transparent background.

[HarshitOnGitHub]

@shubham-padia FYI. This is completely raw as I was doing some testing on the code and haven't cleaned up.

[zulipbot]

Heads up @HarshitOnGitHub, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[timabbott]

Closing, since we fixed #10351 another way, and our research suggests that getting Pillow to not have this sort of problem when resizing animated emoji is unlikely to work. It may be worth reporting a bug to upstream Pillow, though, since if future improvements to Pillow make resizing just work, that might let us switch to a better solution. @HarshitOnGitHub would you be up for doing that? There have to be other Python-based projects interested in resizing animated GIFs without breaking transparency, so this seems like something Pillow upstream might have some interest in...

[HarshitOnGitHub]

@timabbott Sorry for the super-late follow up here, looks like there is already an issue for this: python-pillow/Pillow#3357