add line height to TextFont (#16614)

# Objective

- Allow users to customize the line height of text.
- Implements #16085

## Solution

- Add a `line_height` field to `TextFont` to feed into `cosmic_text`'s
`Metrics`.

## Testing

- Tested on my own game, and worked exactly as I wanted.
- My game is only 2D, so I only tested `Text2d`. `Text` still needs
tested, but I imagine it'll work fine.
- An example is available
[here](https://code.cartoon-aa.xyz/Cyborus/custom-line-height-example)

---

## Showcase

<details>
  <summary>Click to view showcase</summary>

With font:
```rust
TextFont {
    font: /* unimportant */,
    font_size: 16.0,
    line_height: None,
    ..default()
}
```


![image](https://github.com/user-attachments/assets/d12d8334-72ae-44b4-9b2e-993bbfd19da6)

With font:
```rust
TextFont {
    font: /* unimportant */,
    font_size: 16.0,
    line_height: Some(16.0),
    ..default()
}
```


![image](https://github.com/user-attachments/assets/6bc843b0-b633-4c30-bf77-6bbad774c1e5)

</details>

## Migration Guide

`TextFont` now has a `line_height` field. Any instantiation of
`TextFont` that doesn't have `..default()` will need to add this field.

crates/bevy_text/src/lib.rs

@@ -110,6 +110,7 @@ impl Plugin for TextPlugin {
         app.init_asset::<Font>()
             .register_type::<Text2d>()
             .register_type::<TextFont>()
+            .register_type::<LineHeight>()
             .register_type::<TextColor>()
             .register_type::<TextSpan>()
             .register_type::<TextBounds>()

crates/bevy_text/src/pipeline.rs

@@ -98,6 +98,7 @@ impl TextPipeline {
         // Collect span information into a vec. This is necessary because font loading requires mut access
         // to FontSystem, which the cosmic-text Buffer also needs.
         let mut font_size: f32 = 0.;
+        let mut line_height: f32 = 0.0;
         let mut spans: Vec<(usize, &str, &TextFont, FontFaceInfo, Color)> =
             core::mem::take(&mut self.spans_buffer)
                 .into_iter()
@@ -130,6 +131,7 @@ impl TextPipeline {
 
             // Get max font size for use in cosmic Metrics.
             font_size = font_size.max(text_font.font_size);
+            line_height = line_height.max(text_font.line_height.eval(text_font.font_size));
 
             // Load Bevy fonts into cosmic-text's font system.
             let face_info = load_font_to_fontdb(
@@ -146,7 +148,6 @@ impl TextPipeline {
             spans.push((span_index, span, text_font, face_info, color));
         }
 
-        let line_height = font_size * 1.2;
         let mut metrics = Metrics::new(font_size, line_height).scale(scale_factor as f32);
         // Metrics of 0.0 cause `Buffer::set_metrics` to panic. We hack around this by 'falling
         // through' to call `Buffer::set_rich_text` with zero spans so any cached text will be cleared without
@@ -486,7 +487,13 @@ fn get_attrs<'a>(
         .stretch(face_info.stretch)
         .style(face_info.style)
         .weight(face_info.weight)
-        .metrics(Metrics::relative(text_font.font_size, 1.2).scale(scale_factor as f32))
+        .metrics(
+            Metrics {
+                font_size: text_font.font_size,
+                line_height: text_font.line_height.eval(text_font.font_size),
+            }
+            .scale(scale_factor as f32),
+        )
         .color(cosmic_text::Color(color.to_linear().as_u32()));
     attrs
 }

crates/bevy_text/src/text.rs

@@ -286,6 +286,11 @@ pub struct TextFont {
     /// A new font atlas is generated for every combination of font handle and scaled font size
     /// which can have a strong performance impact.
     pub font_size: f32,
+    /// The vertical height of a line of text, from the top of one line to the top of the
+    /// next.
+    ///
+    /// Defaults to `LineHeight::RelativeToFont(1.2)`
+    pub line_height: LineHeight,
     /// The antialiasing method to use when rendering text.
     pub font_smoothing: FontSmoothing,
 }
@@ -325,11 +330,39 @@ impl Default for TextFont {
         Self {
             font: Default::default(),
             font_size: 20.0,
+            line_height: LineHeight::default(),
             font_smoothing: Default::default(),
         }
     }
 }
 
+/// Specifies the height of each line of text for `Text` and `Text2d`
+///
+/// Default is 1.2x the font size
+#[derive(Debug, Clone, Copy, Reflect)]
+#[reflect(Debug)]
+pub enum LineHeight {
+    /// Set line height to a specific number of pixels
+    Px(f32),
+    /// Set line height to a multiple of the font size
+    RelativeToFont(f32),
+}
+
+impl LineHeight {
+    pub(crate) fn eval(self, font_size: f32) -> f32 {
+        match self {
+            LineHeight::Px(px) => px,
+            LineHeight::RelativeToFont(scale) => scale * font_size,
+        }
+    }
+}
+
+impl Default for LineHeight {
+    fn default() -> Self {
+        LineHeight::RelativeToFont(1.2)
+    }
+}
+
 /// The color of the text for this section.
 #[derive(Component, Copy, Clone, Debug, Deref, DerefMut, Reflect)]
 #[reflect(Component, Default, Debug)]

examples/dev_tools/fps_overlay.rs

@@ -26,6 +26,7 @@ fn main() {
                         font: default(),
                         // We could also disable font smoothing,
                         font_smoothing: FontSmoothing::default(),
+                        ..default()
                     },
                     // We can also change color of the overlay
                     text_color: OverlayColor::GREEN,