Public access to font bytes

[romnn]

Thanks for your work on this crate!

I am using it with notan and due to different egui versions in my dependency tree the egui::FontData i get from font_data() does not have the right type.

This PR adds public access to the font bytes so i can construct the notan::egui::FontData myself with the correct type.

This change does not affect the current design but additionally enabled working with multiple egui versions.

Best, Roman