Round sub pixel font sizes (#22646)

# Objective

Sub-pixel font sizes don't provide much benefit with bitmap fonts and
rounding them to the nearest pixel massively reduces the number of font
atlases when a text entity's font size is naively interpolated.

fixes #22626

## Solution

Round the font sizes before adding them to the Cosmic Text `Attrs`.

## Testing

```
cargo run --example animated_ui
```

In main, the window is blank and it rapidly generates gigabytes of font
atlase images.

With this PR, the text is visible and animated, and memory usage remains
stable.

Author: ickshonpe

crates/bevy_text/src/pipeline.rs

@@ -592,19 +592,22 @@ fn get_attrs<'a>(
     family: Family<'a>,
     scale_factor: f64,
 ) -> Attrs<'a> {
+    let font_size = (text_font.font_size * scale_factor as f32).round();
+    let line_height = match line_height {
+        LineHeight::Px(px) => px * scale_factor as f32,
+        LineHeight::RelativeToFont(s) => s * font_size,
+    };
+
     Attrs::new()
         .metadata(span_index)
         .family(family)
         .stretch(text_font.width.into())
         .style(text_font.style.into())
         .weight(text_font.weight.into())
-        .metrics(
-            Metrics {
-                font_size: text_font.font_size,
-                line_height: line_height.eval(text_font.font_size),
-            }
-            .scale(scale_factor as f32),
-        )
+        .metrics(Metrics {
+            font_size,
+            line_height,
+        })
         .font_features((&text_font.font_features).into())
 }
 

crates/bevy_text/src/text.rs

@@ -368,8 +368,9 @@ pub struct TextFont {
     pub font: FontSource,
     /// The vertical height of rasterized glyphs in the font atlas in pixels.
     ///
-    /// This is multiplied by the window scale factor and `UiScale`, but not the text entity
-    /// transform or camera projection.
+    /// This is multiplied by the window scale factor and `UiScale`, but not the text entity's
+    /// transform or camera projection. Then, the scaled font size is rounded to the nearest pixel
+    /// to produce the final font size used during glyph layout.
     ///
     /// A new font atlas is generated for every combination of font handle and scaled font size
     /// which can have a strong performance impact.
@@ -794,15 +795,6 @@ pub enum LineHeight {
     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)