Put font data into Arc to reduce memory consumption (#5276)

egui never accesses the `FontDefinitions`' member fields mutably, except
in `fonts_tweak_ui` where it cloned the `FontDefinitions` object anyway.

This patch reduces system memory consumption for shared font
definitions.
And also removes some overhead from copying (e.g. for the per
`pixel_per_points` font atlas)

Also it allows to keep a copy of the font definitions outside of egui.

In my App that uses international fonts:
Before:

![image](https://github.com/user-attachments/assets/f8dfb4f4-a21c-447c-8cf9-83025ad6e960)

New:

![image](https://github.com/user-attachments/assets/9f297fbd-e620-4b7e-a32a-65073ee805ed)


Note: If `Arc` is not wanted, then it could ofc be abstracted away.

I know this is quite a breaking change API wise, but would like to hear
your opinion.

examples/custom_font/src/main.rs

@@ -48,9 +48,9 @@ fn replace_fonts(ctx: &egui::Context) {
     // .ttf and .otf files supported.
     fonts.font_data.insert(
         "my_font".to_owned(),
-        egui::FontData::from_static(include_bytes!(
+        std::sync::Arc::new(egui::FontData::from_static(include_bytes!(
             "../../../crates/epaint_default_fonts/fonts/Hack-Regular.ttf"
-        )),
+        ))),
     );
 
     // Put my font first (highest priority) for proportional text: