webp generation fails on Linux with Pillow 9.5.0 (key error) incl. workaround

Hi!

Might be specific to my system, but in case this affects more people, here's my workaround/ suggestion for a potential fix.

I had issues with generating WEBP files (if selected as an output/preview format, or as the thumbnail of the infinite image browser extension), resulting in a key error.

Apart from installing libwebp-dev (system package via apt) I had to update Pillow to 10.4.0 to get it working.

As forge would check the requirements file and downgrade pillow to 9.5.0 on every start, I am now using the --skip-install flag (rather than editing the requirements, as that messes with the git pull I run on every start).

So far, I haven't experienced any issues running the new version of Pillow, so it might be worth just updating the version in the requirements file.

Sidenote: Forge has hardcoded versions on every package except pytorch, base Webui has far fewer (protobuf for one), so version updates could probably be done on other packages as well.

Start by locating the requirements file that pins Pillow to 9.5.0 and review how Forge installs it on startup. Reproduce WEBP generation on Linux with the reported dependency setup, then verify that the selected Pillow version avoids the key error without breaking startup installation.

Written by the indexing model from the issue text.