Text2d scalefactor change detection fix (#16264)

# Objective 

Text2d doesn't respond to changes to the window scalefactor.

Fixes #16223

## Solution 

In `update_text2d_layout` store the previous scale factor in a `Local`
instead and check against the current scale factor to detect changes.

It seems like previously the text wasn't updated because of a bug with
the `WindowScaleFactorChanged` event and it isn't emitted after changes
to the scale factor. That needs to be looked into, but this will work
for now.

## Testing

Really simple app that draws a big message in the middle of the window:

```
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2d);
    commands.spawn((
        Text2d::new("Hello"),
        TextFont {
            font_size: 400.,
            ..Default::default()
        },
    ));
}
```

Looks fine:
<img width="500" alt="hello1"
src="https://github.com/user-attachments/assets/5320746b-687e-4682-9e4c-bc43ab7ff9d3">

On main, after changing the monitor's scale factor:
<img width="500" alt="hello2"
src="https://github.com/user-attachments/assets/486cea16-fc44-4d66-9468-6f68905d4196">


With this PR the text maintains the same size and position after the
scale factor is changed.

crates/bevy_text/src/text2d.rs

@@ -11,7 +11,6 @@ use bevy_ecs::component::Component;
 use bevy_ecs::{
     change_detection::{DetectChanges, Ref},
     entity::Entity,
-    event::EventReader,
     prelude::{ReflectComponent, With},
     query::{Changed, Without},
     system::{Commands, Local, Query, Res, ResMut},
@@ -30,7 +29,7 @@ use bevy_sprite::{Anchor, ExtractedSprite, ExtractedSprites, SpriteSource, Textu
 use bevy_transform::components::Transform;
 use bevy_transform::prelude::GlobalTransform;
 use bevy_utils::HashSet;
-use bevy_window::{PrimaryWindow, Window, WindowScaleFactorChanged};
+use bevy_window::{PrimaryWindow, Window};
 
 /// [`Text2dBundle`] was removed in favor of required components.
 /// The core component is now [`Text2d`] which can contain a single text segment.
@@ -235,12 +234,12 @@ pub fn extract_text2d_sprite(
 /// It does not modify or observe existing ones.
 #[allow(clippy::too_many_arguments)]
 pub fn update_text2d_layout(
+    mut last_scale_factor: Local<f32>,
     // Text items which should be reprocessed again, generally when the font hasn't loaded yet.
     mut queue: Local<HashSet<Entity>>,
     mut textures: ResMut<Assets<Image>>,
     fonts: Res<Assets<Font>>,
     windows: Query<&Window, With<PrimaryWindow>>,
-    mut scale_factor_changed: EventReader<WindowScaleFactorChanged>,
     mut texture_atlases: ResMut<Assets<TextureAtlasLayout>>,
     mut font_atlas_sets: ResMut<FontAtlasSets>,
     mut text_pipeline: ResMut<TextPipeline>,
@@ -255,9 +254,6 @@ pub fn update_text2d_layout(
     mut font_system: ResMut<CosmicFontSystem>,
     mut swash_cache: ResMut<SwashCache>,
 ) {
-    // We need to consume the entire iterator, hence `last`
-    let factor_changed = scale_factor_changed.read().last().is_some();
-
     // TODO: Support window-independent scaling: https://github.com/bevyengine/bevy/issues/5621
     let scale_factor = windows
         .get_single()
@@ -266,6 +262,9 @@ pub fn update_text2d_layout(
 
     let inverse_scale_factor = scale_factor.recip();
 
+    let factor_changed = *last_scale_factor != scale_factor;
+    *last_scale_factor = scale_factor;
+
     for (entity, block, bounds, text_layout_info, mut computed) in &mut text_query {
         if factor_changed
             || computed.needs_rerender()
@@ -359,7 +358,7 @@ mod tests {
 
     use bevy_app::{App, Update};
     use bevy_asset::{load_internal_binary_asset, Handle};
-    use bevy_ecs::{event::Events, schedule::IntoSystemConfigs};
+    use bevy_ecs::schedule::IntoSystemConfigs;
 
     use crate::{detect_text_needs_rerender, TextIterScratch};
 
@@ -374,7 +373,6 @@ mod tests {
             .init_resource::<Assets<Image>>()
             .init_resource::<Assets<TextureAtlasLayout>>()
             .init_resource::<FontAtlasSets>()
-            .init_resource::<Events<WindowScaleFactorChanged>>()
             .init_resource::<TextPipeline>()
             .init_resource::<CosmicFontSystem>()
             .init_resource::<SwashCache>()