Finally got nix build to work and thus even make the app installable
again now that I've switched to a mostly rust backend. I needed to add
cargoSetupHook and rustPlatform to the buildInputs.
Updated a lot of things to include ffmpeg_6-full so that we can use
some of the latest ffmpeg features and include ffmpeg-next in our rust
project.
Then some of the environment pieces were missing for ffmpeg-next to build.
This object is still unfinished, but it will later serve as our main
object to connect to for the slides that are shown on screen. All of
this needed some heavy tweaking and I still need to learn more about
rust, but the beginnings are there and it will be worth it to have the
safety and speed that rust provides
