Fix panics in scene_viewer and audio_control (bevyengine#16983)

# Objective

Fixes bevyengine#16978

While testing, discovered that the morph weight interface in
`scene_viewer` has been broken for a while (panics when loaded model has
morph weights), probably since bevyengine#15591. Fixed that too.

While testing, saw example text in morph interface with [wrong
padding](https://bevyengine.org/learn/contribute/helping-out/creating-examples/#visual-guidelines).
Fixed that too. Left the small font size because there may be a lot of
morphs to display, so that seems intentional.

## Solution

Use normal queries and bail early

## Testing

Morph interface can be tested with
```
cargo run --example scene_viewer assets/models/animated/MorphStressTest.gltf
```

## Discussion

I noticed that this fix is different than what is happening in bevyengine#16976.
Feel free to discard this for an alternative fix. I opened this anyway
to document the issue with morph weight display.

This is on top of bevyengine#16966 which is required to test.

---------

Co-authored-by: François Mockers <francois.mockers@vleue.com>
Co-authored-by: François Mockers <mockersf@gmail.com>

examples/audio/audio_control.rs

@@ -34,29 +34,48 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
 #[derive(Component)]
 struct MyMusic;
 
-fn update_speed(sink: Single<&AudioSink, With<MyMusic>>, time: Res<Time>) {
+fn update_speed(music_controller: Query<&AudioSink, With<MyMusic>>, time: Res<Time>) {
+    let Ok(sink) = music_controller.get_single() else {
+        return;
+    };
+
     sink.set_speed((ops::sin(time.elapsed_secs() / 5.0) + 1.0).max(0.1));
 }
 
-fn pause(keyboard_input: Res<ButtonInput<KeyCode>>, sink: Single<&AudioSink, With<MyMusic>>) {
+fn pause(
+    keyboard_input: Res<ButtonInput<KeyCode>>,
+    music_controller: Query<&AudioSink, With<MyMusic>>,
+) {
+    let Ok(sink) = music_controller.get_single() else {
+        return;
+    };
+
     if keyboard_input.just_pressed(KeyCode::Space) {
         sink.toggle_playback();
     }
 }
 
 fn mute(
     keyboard_input: Res<ButtonInput<KeyCode>>,
-    mut sink: Single<&mut AudioSink, With<MyMusic>>,
+    mut music_controller: Query<&mut AudioSink, With<MyMusic>>,
 ) {
+    let Ok(mut sink) = music_controller.get_single_mut() else {
+        return;
+    };
+
     if keyboard_input.just_pressed(KeyCode::KeyM) {
         sink.toggle_mute();
     }
 }
 
 fn volume(
     keyboard_input: Res<ButtonInput<KeyCode>>,
-    mut sink: Single<&mut AudioSink, With<MyMusic>>,
+    mut music_controller: Query<&mut AudioSink, With<MyMusic>>,
 ) {
+    let Ok(mut sink) = music_controller.get_single_mut() else {
+        return;
+    };
+
     if keyboard_input.just_pressed(KeyCode::Equal) {
         let current_volume = sink.volume();
         sink.set_volume(current_volume + 0.1);

examples/helpers/camera_controller.rs

@@ -109,11 +109,13 @@ fn run_camera_controller(
     key_input: Res<ButtonInput<KeyCode>>,
     mut toggle_cursor_grab: Local<bool>,
     mut mouse_cursor_grab: Local<bool>,
-    query: Single<(&mut Transform, &mut CameraController), With<Camera>>,
+    mut query: Query<(&mut Transform, &mut CameraController), With<Camera>>,
 ) {
     let dt = time.delta_secs();
 
-    let (mut transform, mut controller) = query.into_inner();
+    let Ok((mut transform, mut controller)) = query.get_single_mut() else {
+        return;
+    };
 
     if !controller.initialized {
         let (yaw, pitch, _roll) = transform.rotation.to_euler(EulerRot::YXZ);

examples/tools/scene_viewer/morph_viewer_plugin.rs

@@ -12,6 +12,8 @@ use crate::scene_viewer_plugin::SceneHandle;
 use bevy::prelude::*;
 use std::fmt;
 
+const FONT_SIZE: f32 = 13.0;
+
 const WEIGHT_PER_SECOND: f32 = 0.8;
 const ALL_MODIFIERS: &[KeyCode] = &[KeyCode::ShiftLeft, KeyCode::ControlLeft, KeyCode::AltLeft];
 const AVAILABLE_KEYS: [MorphKey; 56] = [
@@ -122,8 +124,8 @@ impl fmt::Display for Target {
     }
 }
 impl Target {
-    fn text_span(&self, key: &str, style: TextFont) -> (String, TextFont) {
-        (format!("[{key}] {self}\n"), style)
+    fn text_span(&self, key: &str, style: TextFont) -> (TextSpan, TextFont) {
+        (TextSpan::new(format!("[{key}] {self}\n")), style)
     }
     fn new(
         entity_name: Option<&Name>,
@@ -178,13 +180,18 @@ impl MorphKey {
 }
 fn update_text(
     controls: Option<ResMut<WeightsControl>>,
-    text: Single<Entity, With<Text>>,
+    texts: Query<Entity, With<Text>>,
     morphs: Query<&MorphWeights>,
     mut writer: TextUiWriter,
 ) {
     let Some(mut controls) = controls else {
         return;
     };
+
+    let Ok(text) = texts.get_single() else {
+        return;
+    };
+
     for (i, target) in controls.weights.iter_mut().enumerate() {
         let Ok(weights) = morphs.get(target.entity) else {
             continue;
@@ -196,7 +203,8 @@ fn update_text(
             target.weight = actual_weight;
         }
         let key_name = &AVAILABLE_KEYS[i].name;
-        *writer.text(*text, i + 3) = format!("[{key_name}] {target}\n");
+
+        *writer.text(text, i + 3) = format!("[{key_name}] {target}\n");
     }
 }
 fn update_morphs(
@@ -254,12 +262,12 @@ fn detect_morphs(
     }
     detected.truncate(AVAILABLE_KEYS.len());
     let style = TextFont {
-        font_size: 13.0,
+        font_size: FONT_SIZE,
         ..default()
     };
     let mut spans = vec![
-        ("Morph Target Controls\n".into(), style.clone()),
-        ("---------------\n".into(), style.clone()),
+        (TextSpan::new("Morph Target Controls\n"), style.clone()),
+        (TextSpan::new("---------------\n"), style.clone()),
     ];
     let target_to_text =
         |(i, target): (usize, &Target)| target.text_span(AVAILABLE_KEYS[i].name, style.clone());
@@ -270,14 +278,15 @@ fn detect_morphs(
             Text::default(),
             Node {
                 position_type: PositionType::Absolute,
-                top: Val::Px(10.0),
-                left: Val::Px(10.0),
+                top: Val::Px(12.0),
+                left: Val::Px(12.0),
                 ..default()
             },
         ))
         .with_children(|p| {
-            p.spawn((TextSpan::new("Morph Target Controls\n"), style.clone()));
-            p.spawn((TextSpan::new("---------------\n"), style));
+            for span in spans {
+                p.spawn(span);
+            }
         });
 }