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.

crates/egui/src/context.rs

@@ -597,7 +597,9 @@ impl ContextImpl {
                         FontPriority::Lowest => fam.push(font.name.clone()),
                     }
                 }
-                self.font_definitions.font_data.insert(font.name, font.data);
+                self.font_definitions
+                    .font_data
+                    .insert(font.name, Arc::new(font.data));
             }
 
             #[cfg(feature = "log")]
@@ -2944,7 +2946,9 @@ impl Context {
 
         for (name, data) in &mut font_definitions.font_data {
             ui.collapsing(name, |ui| {
-                if data.tweak.ui(ui).changed() {
+                let mut tweak = data.tweak;
+                if tweak.ui(ui).changed() {
+                    Arc::make_mut(data).tweak = tweak;
                     changed = true;
                 }
             });

crates/epaint/src/text/fonts.rs

@@ -224,7 +224,11 @@ fn ab_glyph_font_from_font_data(name: &str, data: &FontData) -> ab_glyph::FontAr
 ///
 /// // Install my own font (maybe supporting non-latin characters):
 /// fonts.font_data.insert("my_font".to_owned(),
-///    FontData::from_static(include_bytes!("../../../epaint_default_fonts/fonts/Ubuntu-Light.ttf"))); // .ttf and .otf supported
+///    std::sync::Arc::new(
+///        // .ttf and .otf supported
+///        FontData::from_static(include_bytes!("../../../epaint_default_fonts/fonts/Ubuntu-Light.ttf"))
+///    )
+/// );
 ///
 /// // Put my font first (highest priority):
 /// fonts.families.get_mut(&FontFamily::Proportional).unwrap()
@@ -243,7 +247,7 @@ pub struct FontDefinitions {
     /// List of font names and their definitions.
     ///
     /// `epaint` has built-in-default for these, but you can override them if you like.
-    pub font_data: BTreeMap<String, FontData>,
+    pub font_data: BTreeMap<String, Arc<FontData>>,
 
     /// Which fonts (names) to use for each [`FontFamily`].
     ///
@@ -310,33 +314,36 @@ impl Default for FontDefinitions {
     /// otherwise this is the same as [`Self::empty`].
     #[cfg(feature = "default_fonts")]
     fn default() -> Self {
-        let mut font_data: BTreeMap<String, FontData> = BTreeMap::new();
+        let mut font_data: BTreeMap<String, Arc<FontData>> = BTreeMap::new();
 
         let mut families = BTreeMap::new();
 
-        font_data.insert("Hack".to_owned(), FontData::from_static(HACK_REGULAR));
+        font_data.insert(
+            "Hack".to_owned(),
+            Arc::new(FontData::from_static(HACK_REGULAR)),
+        );
 
         // Some good looking emojis. Use as first priority:
         font_data.insert(
             "NotoEmoji-Regular".to_owned(),
-            FontData::from_static(NOTO_EMOJI_REGULAR).tweak(FontTweak {
+            Arc::new(FontData::from_static(NOTO_EMOJI_REGULAR).tweak(FontTweak {
                 scale: 0.81, // Make smaller
                 ..Default::default()
-            }),
+            })),
         );
 
         font_data.insert(
             "Ubuntu-Light".to_owned(),
-            FontData::from_static(UBUNTU_LIGHT),
+            Arc::new(FontData::from_static(UBUNTU_LIGHT)),
         );
 
         // Bigger emojis, and more. <http://jslegers.github.io/emoji-icon-font/>:
         font_data.insert(
             "emoji-icon-font".to_owned(),
-            FontData::from_static(EMOJI_ICON).tweak(FontTweak {
+            Arc::new(FontData::from_static(EMOJI_ICON).tweak(FontTweak {
                 scale: 0.90, // Make smaller
                 ..Default::default()
-            }),
+            })),
         );
 
         families.insert(
@@ -795,7 +802,7 @@ impl FontImplCache {
     pub fn new(
         atlas: Arc<Mutex<TextureAtlas>>,
         pixels_per_point: f32,
-        font_data: &BTreeMap<String, FontData>,
+        font_data: &BTreeMap<String, Arc<FontData>>,
     ) -> Self {
         let ab_glyph_fonts = font_data
             .iter()

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: