extract_text_shadows camera query fix (#17930)

# Objective

`extract_text_shadows` was still using `UiTargetCamera` and
`DefaultUiCamera` for UI camera resolution, which no longer always
selects the right camera.

To see this modify the last lines of the `multiple_windows` example
from:
```rust
    commands.spawn((
        Text::new("First window"),
        node.clone(),
        // Since we are using multiple cameras, we need to specify which camera UI should be rendered to
        UiTargetCamera(first_window_camera),
    ));

    commands.spawn((
        Text::new("Second window"),
        node,
        UiTargetCamera(second_window_camera),
    ));
```
to:
```rust
    commands
        .spawn((
            node.clone(),
            // Since we are using multiple cameras, we need to specify which camera UI should be rendered to
            UiTargetCamera(first_window_camera),
        ))
        .with_child((Text::new("First window"), TextShadow::default()));

    commands
        .spawn((node, UiTargetCamera(second_window_camera)))
        .with_child((Text::new("Second window"), TextShadow::default()));
```

which results in the shadow that is meant to be displayed for the
"Second Window" label instead being written over the first:

<img width="800" alt="first_window_label"
src="https://github.com/user-attachments/assets/2eebccba-5749-4064-bb1c-e4f25ff0baf7">

## Solution
Remove the `UiTargetCamera` query and the `default_camera` parameter
from `extract_text_shadows` and use `UiCameraMap` with
`ComputedNodeTarget` instead.

## Testing
The `multiple_windows` example for this PR has been updated to add text
shadow to the window labels. You should see that it displays the "Second
Window" label's shadow correctly now.

crates/bevy_ui/src/render/mod.rs

@@ -10,8 +10,7 @@ mod debug_overlay;
 use crate::widget::ImageNode;
 use crate::{
     BackgroundColor, BorderColor, BoxShadowSamples, CalculatedClip, ComputedNode,
-    ComputedNodeTarget, DefaultUiCamera, Outline, ResolvedBorderRadius, TextShadow, UiAntiAlias,
-    UiTargetCamera,
+    ComputedNodeTarget, Outline, ResolvedBorderRadius, TextShadow, UiAntiAlias,
 };
 use bevy_app::prelude::*;
 use bevy_asset::{load_internal_asset, weak_handle, AssetEvent, AssetId, Assets, Handle};
@@ -794,47 +793,42 @@ pub fn extract_text_sections(
 pub fn extract_text_shadows(
     mut commands: Commands,
     mut extracted_uinodes: ResMut<ExtractedUiNodes>,
-    default_ui_camera: Extract<DefaultUiCamera>,
     texture_atlases: Extract<Res<Assets<TextureAtlasLayout>>>,
     uinode_query: Extract<
         Query<(
             Entity,
             &ComputedNode,
+            &ComputedNodeTarget,
             &GlobalTransform,
             &InheritedVisibility,
             Option<&CalculatedClip>,
-            Option<&UiTargetCamera>,
             &TextLayoutInfo,
             &TextShadow,
         )>,
     >,
-    mapping: Extract<Query<RenderEntity>>,
+    camera_map: Extract<UiCameraMap>,
 ) {
     let mut start = extracted_uinodes.glyphs.len();
     let mut end = start + 1;
 
-    let default_ui_camera = default_ui_camera.get();
+    let mut camera_mapper = camera_map.get_mapper();
     for (
         entity,
         uinode,
+        target,
         global_transform,
         inherited_visibility,
         clip,
-        camera,
         text_layout_info,
         shadow,
     ) in &uinode_query
     {
-        let Some(camera_entity) = camera.map(UiTargetCamera::entity).or(default_ui_camera) else {
-            continue;
-        };
-
         // Skip if not visible or if size is set to zero (e.g. when a parent is set to `Display::None`)
         if !inherited_visibility.get() || uinode.is_empty() {
             continue;
         }
 
-        let Ok(extracted_camera_entity) = mapping.get(camera_entity) else {
+        let Some(extracted_camera_entity) = camera_mapper.map(target) else {
             continue;
         };
 

examples/window/multiple_windows.rs

@@ -54,16 +54,15 @@ fn setup_scene(mut commands: Commands, asset_server: Res<AssetServer>) {
         ..default()
     };
 
-    commands.spawn((
-        Text::new("First window"),
+    commands
+        .spawn((
             node.clone(),
             // Since we are using multiple cameras, we need to specify which camera UI should be rendered to
             UiTargetCamera(first_window_camera),
-    ));
+        ))
+        .with_child((Text::new("First window"), TextShadow::default()));
 
-    commands.spawn((
-        Text::new("Second window"),
-        node,
-        UiTargetCamera(second_window_camera),
-    ));
+    commands
+        .spawn((node, UiTargetCamera(second_window_camera)))
+        .with_child((Text::new("Second window"), TextShadow::default()));
 }