Add UI viewport tests to split_screen example

This example shows that only the UI on the highest priority camera
will ever render. Looking at output, you can clearly see that the
draw calls are happening.

examples/3d/split_screen.rs

@@ -47,15 +47,28 @@ fn setup(
     });
 
     // Left Camera
-    commands
+    // BUG: UI viewports seem to clear or prevent each other from drawing so this doesn't render at all.
+    // Changing this to 2 will still display it but then the right UI will not display anymore!
+    let left_camera = commands
         .spawn_bundle(Camera3dBundle {
             transform: Transform::from_xyz(0.0, 200.0, -100.0).looking_at(Vec3::ZERO, Vec3::Y),
+            camera: Camera {
+                // Renders the left camera before the right camera, which has a priority of one.
+                priority: 0,
+                ..default()
+            },
+            camera_3d: Camera3d {
+                // To allow testing priority change, don't clear at all.
+                clear_color: ClearColorConfig::None,
+                ..default()
+            },
             ..default()
         })
-        .insert(LeftCamera);
+        .insert(LeftCamera)
+        .id();
 
     // Right Camera
-    commands
+    let right_camera = commands
         .spawn_bundle(Camera3dBundle {
             transform: Transform::from_xyz(100.0, 100., 150.0).looking_at(Vec3::ZERO, Vec3::Y),
             camera: Camera {
@@ -70,7 +83,31 @@ fn setup(
             },
             ..default()
         })
-        .insert(RightCamera);
+        .insert(RightCamera)
+        .id();
+
+    // Text to be rendered on the left camera
+    commands.spawn_bundle(TextBundle::from_section(
+        "Left camera",
+        TextStyle {
+            font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+            font_size: 40.0,
+            color: Color::BLUE,
+        },
+    ))
+    .insert(UiRootCamera(left_camera));
+
+    // Text to be rendered on the right camera
+    commands.spawn_bundle(
+        TextBundle::from_section(
+        "Right camera",
+        TextStyle {
+            font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+            font_size: 40.0,
+            color: Color::GREEN,
+        },
+    ))
+    .insert(UiRootCamera(right_camera));
 }
 
 #[derive(Component)]