impl `Clone` for `Fonts` (#3737)

Closes https://github.com/emilk/egui/issues/3731

.vscode/settings.json

@@ -32,4 +32,5 @@
         "--all-targets",
         "--all-features",
     ],
+    "rust-analyzer.showUnlinkedFileNotification": false,
 }

crates/epaint/src/text/fonts.rs

@@ -348,6 +348,7 @@ impl FontDefinitions {
 /// If you are using `egui`, use `egui::Context::set_fonts` and `egui::Context::fonts`.
 ///
 /// You need to call [`Self::begin_frame`] and [`Self::font_image_delta`] once every frame.
+#[derive(Clone)]
 pub struct Fonts(Arc<Mutex<FontsAndCache>>);
 
 impl Fonts {