UI text extraction refactor (#17805)

## Objective

There's no need for the `span_index` and `color` variables in
`extract_text_shadows` and `extract_text_sections` and we can remove one
of the span index comparisons since text colors are only set per
section.

## Testing

<img width="454" alt="trace"
src="https://github.com/user-attachments/assets/3109d1df-0817-46c2-9889-0459ac93a42c"
/>

crates/bevy_ui/src/render/mod.rs

@@ -739,9 +739,6 @@ pub fn extract_text_sections(
         let transform = global_transform.affine()
             * bevy_math::Affine3A::from_translation((-0.5 * uinode.size()).extend(0.));
 
-        let mut color = LinearRgba::WHITE;
-        let mut current_span = usize::MAX;
-
         for (
             i,
             PositionedGlyph {
@@ -752,20 +749,6 @@ pub fn extract_text_sections(
             },
         ) in text_layout_info.glyphs.iter().enumerate()
         {
-            if *span_index != current_span {
-                color = text_styles
-                    .get(
-                        computed_block
-                            .entities()
-                            .get(*span_index)
-                            .map(|t| t.entity)
-                            .unwrap_or(Entity::PLACEHOLDER),
-                    )
-                    .map(|text_color| LinearRgba::from(text_color.0))
-                    .unwrap_or_default();
-                current_span = *span_index;
-            }
-
             let rect = texture_atlases
                 .get(&atlas_info.texture_atlas)
                 .unwrap()
@@ -777,8 +760,18 @@ pub fn extract_text_sections(
             });
 
             if text_layout_info.glyphs.get(i + 1).is_none_or(|info| {
-                info.span_index != current_span || info.atlas_info.texture != atlas_info.texture
+                info.span_index != *span_index || info.atlas_info.texture != atlas_info.texture
             }) {
+                let color = text_styles
+                    .get(
+                        computed_block
+                            .entities()
+                            .get(*span_index)
+                            .map(|t| t.entity)
+                            .unwrap_or(Entity::PLACEHOLDER),
+                    )
+                    .map(|text_color| LinearRgba::from(text_color.0))
+                    .unwrap_or_default();
                 extracted_uinodes.uinodes.push(ExtractedUiNode {
                     render_entity: commands.spawn(TemporaryRenderEntity).id(),
                     stack_index: uinode.stack_index,
@@ -850,7 +843,6 @@ pub fn extract_text_shadows(
                 (-0.5 * uinode.size() + shadow.offset / uinode.inverse_scale_factor()).extend(0.),
             );
 
-        let mut current_span = usize::MAX;
         for (
             i,
             PositionedGlyph {
@@ -861,10 +853,6 @@ pub fn extract_text_shadows(
             },
         ) in text_layout_info.glyphs.iter().enumerate()
         {
-            if *span_index != current_span {
-                current_span = *span_index;
-            }
-
             let rect = texture_atlases
                 .get(&atlas_info.texture_atlas)
                 .unwrap()
@@ -876,7 +864,7 @@ pub fn extract_text_shadows(
             });
 
             if text_layout_info.glyphs.get(i + 1).is_none_or(|info| {
-                info.span_index != current_span || info.atlas_info.texture != atlas_info.texture
+                info.span_index != *span_index || info.atlas_info.texture != atlas_info.texture
             }) {
                 extracted_uinodes.uinodes.push(ExtractedUiNode {
                     render_entity: commands.spawn(TemporaryRenderEntity).id(),