Update dependencies

[emilk]

Update gltf, image, rand and glutin.

The only crate using an old version is egui, and I'll wait with that until I have time to to take a look at #216

[asny]

Thanks for the contribution, again 👍
I couldn't get rand 0.8 to work on web, that's why it's 0.7, but it still uses 0.7 on web (and it's only a dev dependency) so I think it is fine.
Just FYI, I plan to release one of these days 💪