Add control of line height and letter spacing (#3302)

* Add `TextFormat::extra_letter_spacing`

* Add control of line height

* Add to text layout demo

* Move the text layout demo to its own window in the demo app

* Fix doclink

* Better document points vs pixels

* Better documentation and code cleanup

crates/egui_demo_lib/src/demo/text_layout.rs

@@ -0,0 +1,135 @@
+/// Showcase text layout
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(default))]
+pub struct TextLayoutDemo {
+    break_anywhere: bool,
+    max_rows: usize,
+    overflow_character: Option<char>,
+    extra_letter_spacing_pixels: i32,
+    line_height_pixels: u32,
+}
+
+impl Default for TextLayoutDemo {
+    fn default() -> Self {
+        Self {
+            max_rows: 3,
+            break_anywhere: true,
+            overflow_character: Some('…'),
+            extra_letter_spacing_pixels: 0,
+            line_height_pixels: 0,
+        }
+    }
+}
+
+impl super::Demo for TextLayoutDemo {
+    fn name(&self) -> &'static str {
+        "🖹 Text Layout"
+    }
+
+    fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
+        egui::Window::new(self.name())
+            .open(open)
+            .resizable(true)
+            .show(ctx, |ui| {
+                use super::View as _;
+                self.ui(ui);
+            });
+    }
+}
+
+impl super::View for TextLayoutDemo {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        let Self {
+            break_anywhere,
+            max_rows,
+            overflow_character,
+            extra_letter_spacing_pixels,
+            line_height_pixels,
+        } = self;
+
+        use egui::text::LayoutJob;
+
+        let pixels_per_point = ui.ctx().pixels_per_point();
+        let points_per_pixel = 1.0 / pixels_per_point;
+
+        ui.vertical_centered(|ui| {
+            ui.add(crate::egui_github_link_file_line!());
+        });
+
+        ui.add_space(12.0);
+
+        egui::Grid::new("TextLayoutDemo")
+            .num_columns(2)
+            .show(ui, |ui| {
+                ui.label("Max rows:");
+                ui.add(egui::DragValue::new(max_rows));
+                ui.end_row();
+
+                ui.label("Line-break:");
+                ui.horizontal(|ui| {
+                    ui.radio_value(break_anywhere, false, "word boundaries");
+                    ui.radio_value(break_anywhere, true, "anywhere");
+                });
+                ui.end_row();
+
+                ui.label("Overflow character:");
+                ui.horizontal(|ui| {
+                    ui.selectable_value(overflow_character, None, "None");
+                    ui.selectable_value(overflow_character, Some('…'), "…");
+                    ui.selectable_value(overflow_character, Some('—'), "—");
+                    ui.selectable_value(overflow_character, Some('-'), "  -  ");
+                });
+                ui.end_row();
+
+                ui.label("Extra letter spacing:");
+                ui.add(egui::DragValue::new(extra_letter_spacing_pixels).suffix(" pixels"));
+                ui.end_row();
+
+                ui.label("Line height:");
+                ui.horizontal(|ui| {
+                    if ui
+                        .selectable_label(*line_height_pixels == 0, "Default")
+                        .clicked()
+                    {
+                        *line_height_pixels = 0;
+                    }
+                    if ui
+                        .selectable_label(*line_height_pixels != 0, "Custom")
+                        .clicked()
+                    {
+                        *line_height_pixels = (pixels_per_point * 20.0).round() as _;
+                    }
+                    if *line_height_pixels != 0 {
+                        ui.add(egui::DragValue::new(line_height_pixels).suffix(" pixels"));
+                    }
+                });
+                ui.end_row();
+            });
+
+        ui.add_space(12.0);
+
+        egui::ScrollArea::vertical().show(ui, |ui| {
+            let extra_letter_spacing = points_per_pixel * *extra_letter_spacing_pixels as f32;
+            let line_height =
+                (*line_height_pixels != 0).then_some(points_per_pixel * *line_height_pixels as f32);
+
+            let mut job = LayoutJob::single_section(
+                crate::LOREM_IPSUM_LONG.to_owned(),
+                egui::TextFormat {
+                    extra_letter_spacing,
+                    line_height,
+                    ..Default::default()
+                },
+            );
+            job.wrap = egui::text::TextWrapping {
+                max_rows: *max_rows,
+                break_anywhere: *break_anywhere,
+                overflow_character: *overflow_character,
+                ..Default::default()
+            };
+
+            // NOTE: `Label` overrides some of the wrapping settings, e.g. wrap width
+            ui.label(job);
+        });
+    }
+}