Getting mpvacious to work

I'm interested in getting [mpvacious](https://github.com/Ajatt-Tools/mpvacious) to work with this Celluloid Flatpak package. It works properly after a recent commit to the native Celluloid program, but some more work needs to be done to get the Flatpak to work. Before I go ahead and do that work, I wanted to ask if there was any opposition to supporting it.

mpvacious uses multiple scripts rather than a single script; support for this was added in https://github.com/celluloid-player/celluloid/commit/3ad59898b4a47620be3e58fcdf6d75a7631d3afa but has not reached the stable release yet.

The following issues need to be fixed:
1. Support `libopus` and `libwebp` encoding so mpvacious can export images and audio from the video to Anki cards: https://github.com/kekkoudesu/io.github.celluloid_player.Celluloid/issues/1
2. Support copying subtitles to clipboard with `wl-clipboard` so Yomitan can look them up in its dictionaries: https://github.com/kekkoudesu/io.github.celluloid_player.Celluloid/issues/2

Edit: Presumably I can just build `wl-clipboard` and it will work properly inside the Flatpak environment. Need to test.