Fix glyph caching on font variations (#8189)

* Closes https://github.com/emilk/egui/pull/8029

---------

Co-authored-by: lucasmerlin <hi@lucasmerlin.me>

examples/font_variations/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "font_variations"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2024"
+rust-version = "1.92"
+publish = false
+
+[lints]
+workspace = true
+
+
+[dependencies]
+eframe = { workspace = true, features = [
+  "default",
+  "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
+] }
+env_logger = { workspace = true, features = ["auto-color", "humantime"] }

examples/font_variations/src/main.rs

@@ -0,0 +1,129 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+#![expect(rustdoc::missing_crate_level_docs)] // it's an example
+
+use eframe::egui;
+use eframe::epaint::text::{FontInsert, InsertFontFamily};
+
+fn main() -> eframe::Result {
+    env_logger::init();
+    let options = eframe::NativeOptions {
+        viewport: egui::ViewportBuilder::default().with_inner_size([600.0, 500.0]),
+        ..Default::default()
+    };
+    eframe::run_native(
+        "egui example: font variations",
+        options,
+        Box::new(|cc| Ok(Box::new(MyApp::new(cc)))),
+    )
+}
+
+struct MyApp {
+    /// Weight axis (wght): 300..1000
+    weight: f32,
+
+    /// Casual axis (CASL): 0..1
+    casual: f32,
+
+    /// Monospace axis (MONO): 0..1
+    mono: f32,
+
+    /// Slant axis (slnt): -15..0
+    slant: f32,
+
+    /// Cursive axis (CRSV): 0..1
+    cursive: f32,
+
+    preview_text: String,
+    font_size: f32,
+}
+
+impl MyApp {
+    fn new(cc: &eframe::CreationContext<'_>) -> Self {
+        cc.egui_ctx.add_font(FontInsert::new(
+            "Recursive",
+            egui::FontData::from_static({
+                #[expect(clippy::large_include_file, reason = "intentional for the example")]
+                {
+                    include_bytes!("../data/Recursive-VariableFont.ttf")
+                }
+            }),
+            vec![
+                InsertFontFamily {
+                    family: egui::FontFamily::Proportional,
+                    priority: egui::epaint::text::FontPriority::Highest,
+                },
+                InsertFontFamily {
+                    family: egui::FontFamily::Monospace,
+                    priority: egui::epaint::text::FontPriority::Highest,
+                },
+            ],
+        ));
+
+        Self {
+            weight: 400.0,
+            casual: 0.0,
+            mono: 0.0,
+            slant: 0.0,
+            cursive: 0.5,
+            preview_text: "The quick brown fox jumps over the lazy dog.\n\
+                           ABCDEFGHIJKLMNOPQRSTUVWXYZ\n\
+                           abcdefghijklmnopqrstuvwxyz\n\
+                           0123456789 !@#$%^&*()"
+                .to_owned(),
+            font_size: 24.0,
+        }
+    }
+}
+
+impl eframe::App for MyApp {
+    fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show_inside(ui, |ui| {
+            ui.heading("Font Variations (Recursive)");
+            ui.add_space(4.0);
+
+            egui::Grid::new("variation_sliders")
+                .num_columns(2)
+                .spacing([16.0, 8.0])
+                .show(ui, |ui| {
+                    ui.label("Weight (wght):");
+                    ui.add(egui::Slider::new(&mut self.weight, 300.0..=1000.0));
+                    ui.end_row();
+
+                    ui.label("Casual (CASL):");
+                    ui.add(egui::Slider::new(&mut self.casual, 0.0..=1.0));
+                    ui.end_row();
+
+                    ui.label("Monospace (MONO):");
+                    ui.add(egui::Slider::new(&mut self.mono, 0.0..=1.0));
+                    ui.end_row();
+
+                    ui.label("Slant (slnt):");
+                    ui.add(egui::Slider::new(&mut self.slant, -15.0..=0.0));
+                    ui.end_row();
+
+                    ui.label("Cursive (CRSV):");
+                    ui.add(egui::Slider::new(&mut self.cursive, 0.0..=1.0));
+                    ui.end_row();
+
+                    ui.label("Font size:");
+                    ui.add(egui::Slider::new(&mut self.font_size, 8.0..=80.0));
+                    ui.end_row();
+                });
+
+            ui.separator();
+
+            let rich = egui::RichText::new(&self.preview_text)
+                .size(self.font_size)
+                .variation("wght", self.weight)
+                .variation("CASL", self.casual)
+                .variation("MONO", self.mono)
+                .variation("slnt", self.slant)
+                .variation("CRSV", self.cursive);
+
+            ui.label(rich);
+
+            ui.add_space(8.0);
+            ui.text_edit_multiline(&mut self.preview_text);
+        });
+    }
+}