Fix text alignment for unbounded text (#17270)

Fixes #16783

Works around a `cosmic-text` bug or limitation by triggering a re-layout
with the calculated width from the first layout run. See linked issue.

Credit to @ickshonpe for the clever solution.

This has a significant performance impact only on unbounded text that
are not `JustifyText::Left`, which is still a bit of a bummer because
text2d performance in 0.15.1 is already not great. But this seems better
than alignment not working.

||many_text2d nfc re|many_text2d nfc re center|
|-|-|-|
|unbounded-layout-no-fix|3.06|3.10|
|unbounded-layout-fix|3.05 ⬜ -0.2%|2.71 🟥 -12.5%|

I added a centered text to the `text2d` example.

`cargo run --example text2d`

We should look at other text examples and stress tests. I haven't tested
as thoroughly as I would like, so help testing that this doesn't break
something in UI would be appreciated.

crates/bevy_text/src/pipeline.rs

@@ -193,6 +193,14 @@ impl TextPipeline {
         }
         buffer.shape_until_scroll(font_system, false);
 
+        // Workaround for alignment not working for unbounded text.
+        // See https://github.com/pop-os/cosmic-text/issues/343
+        if bounds.width.is_none() && justify != JustifyText::Left {
+            let dimensions = buffer_dimensions(buffer);
+            // `set_size` causes a re-layout to occur.
+            buffer.set_size(font_system, Some(dimensions.x), bounds.height);
+        }
+
         // Recover the spans buffer.
         spans.clear();
         self.spans_buffer = spans

examples/2d/text2d.rs

@@ -111,10 +111,11 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
 
     // Demonstrate font smoothing off
     commands.spawn((
-        Text2d::new("FontSmoothing::None"),
+        Text2d::new("This text has\nFontSmoothing::None\nAnd JustifyText::Center"),
         slightly_smaller_text_font
             .clone()
             .with_font_smoothing(FontSmoothing::None),
+        TextLayout::new_with_justify(JustifyText::Center),
         Transform::from_translation(Vec3::new(-400.0, -250.0, 0.0)),
     ));