Is there a better way to do pick-exactly-one for some optional dependencies in eframe/Cargo.toml?

That's the main reason why I didn't address this in the main PR, I'm not really sure how to do that elegantly.
Still not sure, as a last resort we could do separate flags for egui_glow, like glow_persistence etc, but that's quite a bad compromise to make and would be very confusing for users.

I think this is a good enough start