Zero fontsize panic workaround (#15371)

# Objective

- Fix https://github.com/bevyengine/bevy/issues/15366. `cosmic-text`
buffers refuse to function if the `Metrics` font size is zero.

## Solution

- Trick `cosmic-text` into clearing its internal buffer when the largest
font size of segments is zero by sending it no spans and a tiny
`Metrics::font_size` and `Metrics::line_height`.

## Testing

- [x] Fixes @brandon-reinhart 's bug.

crates/bevy_text/src/pipeline.rs

@@ -90,7 +90,12 @@ impl TextPipeline {
                 .ok_or(TextError::NoSuchFont)?;
         }
         let line_height = font_size * 1.2;
-        let metrics = Metrics::new(font_size, line_height).scale(scale_factor as f32);
+        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
+        // deallocating the buffer.
+        metrics.font_size = metrics.font_size.max(0.000001);
+        metrics.line_height = metrics.line_height.max(0.000001);
 
         // Load Bevy fonts into cosmic-text's font system.
         // This is done as as separate pre-pass to avoid borrow checker issues
@@ -108,6 +113,8 @@ impl TextPipeline {
             .into_iter()
             .map(|_| -> (&str, Attrs) { unreachable!() })
             .collect();
+        // `metrics.font_size` hack continued: ignore all spans when scale_factor is zero.
+        if scale_factor > 0.0 {
             spans.extend(
                 sections
                     .iter()
@@ -126,6 +133,7 @@ impl TextPipeline {
                         )
                     }),
             );
+        }
         let spans_iter = spans.iter().copied();
 
         buffer.set_metrics(font_system, metrics);