Avoid reallocating spans buffer in TextPipeline (#15012)

# Objective

- Don't reallocate the spans vector every time TextPipeline updates a
buffer.

## Solution

- Cache the spans buffer in `TextPipeline`. This is possible through
some [rust
magic](https://users.rust-lang.org/t/how-to-cache-a-vectors-capacity/94478/10).

crates/bevy_text/src/pipeline.rs

@@ -51,6 +51,10 @@ pub struct TextPipeline {
  ///
  /// See [`cosmic_text::SwashCache`] for more information.
  swash_cache: SwashCache,
+ /// Buffered vec for collecting spans.
+ ///
+ /// See [this dark magic](https://users.rust-lang.org/t/how-to-cache-a-vectors-capacity/94478/10).
+ spans_buffer: Vec<(&'static str, Attrs<'static>)>,
 }
 
 impl TextPipeline {
@@ -95,23 +99,29 @@ impl TextPipeline {
  // The section index is stored in the metadata of the spans, and could be used
  // to look up the section the span came from and is not used internally
  // in cosmic-text.
- let spans: Vec<(&str, Attrs)> = sections
- .iter()
- .enumerate()
- .filter(|(_section_index, section)| section.style.font_size > 0.0)
- .map(|(section_index, section)| {
- (
- &section.value[..],
- get_attrs(
- section,
- section_index,
- font_system,
- &self.map_handle_to_font_id,
- scale_factor,
- ),
- )
- })
+ let mut spans: Vec<(&str, Attrs)> = std::mem::take(&mut self.spans_buffer)
+ .into_iter()
+ .map(|_| -> (&str, Attrs) { unreachable!() })
  .collect();
+ spans.extend(
+ sections
+ .iter()
+ .enumerate()
+ .filter(|(_section_index, section)| section.style.font_size > 0.0)
+ .map(|(section_index, section)| {
+ (
+ &section.value[..],
+ get_attrs(
+ section,
+ section_index,
+ font_system,
+ &self.map_handle_to_font_id,
+ scale_factor,
+ ),
+ )
+ }),
+ );
+ let spans_iter = spans.iter().copied();
 
  buffer.set_metrics(font_system, metrics);
  buffer.set_size(font_system, bounds.width, bounds.height);
@@ -126,7 +136,7 @@ impl TextPipeline {
  },
  );
 
- buffer.set_rich_text(font_system, spans, Attrs::new(), Shaping::Advanced);
+ buffer.set_rich_text(font_system, spans_iter, Attrs::new(), Shaping::Advanced);
 
  // PERF: https://github.com/pop-os/cosmic-text/issues/166:
  // Setting alignment afterwards appears to invalidate some layouting performed by `set_text` which is presumably not free?
@@ -135,6 +145,13 @@ impl TextPipeline {
  }
  buffer.shape_until_scroll(font_system, false);
 
+ // Recover the spans buffer.
+ spans.clear();
+ self.spans_buffer = spans
+ .into_iter()
+ .map(|_| -> (&'static str, Attrs<'static>) { unreachable!() })
+ .collect();
+
  Ok(())
  }