Refactor 3d_viewport_to_world example with let chains (#20071)

# Objective

Make use of let chains to reduce LoC where we previously used let else
to cut down on indentation. Best of both worlds.

Author: tim-blackbird

examples/3d/3d_viewport_to_world.rs

@@ -13,41 +13,30 @@ fn main() {
 fn draw_cursor(
     camera_query: Single<(&Camera, &GlobalTransform)>,
     ground: Single<&GlobalTransform, With<Ground>>,
-    windows: Query<&Window>,
+    window: Single<&Window>,
     mut gizmos: Gizmos,
 ) {
-    let Ok(windows) = windows.single() else {
-        return;
-    };
-
     let (camera, camera_transform) = *camera_query;
 
-    let Some(cursor_position) = windows.cursor_position() else {
-        return;
-    };
-
-    // Calculate a ray pointing from the camera into the world based on the cursor's position.
-    let Ok(ray) = camera.viewport_to_world(camera_transform, cursor_position) else {
-        return;
-    };
-
-    // Calculate if and where the ray is hitting the ground plane.
-    let Some(distance) =
-        ray.intersect_plane(ground.translation(), InfinitePlane3d::new(ground.up()))
-    else {
-        return;
-    };
-    let point = ray.get_point(distance);
+    if let Some(cursor_position) = window.cursor_position()
+        // Calculate a ray pointing from the camera into the world based on the cursor's position.
+        && let Ok(ray) = camera.viewport_to_world(camera_transform, cursor_position)
+        // Calculate if and at what distance the ray is hitting the ground plane.
+        && let Some(distance) =
+            ray.intersect_plane(ground.translation(), InfinitePlane3d::new(ground.up()))
+    {
+        let point = ray.get_point(distance);
 
-    // Draw a circle just above the ground plane at that position.
-    gizmos.circle(
-        Isometry3d::new(
-            point + ground.up() * 0.01,
-            Quat::from_rotation_arc(Vec3::Z, ground.up().as_vec3()),
-        ),
-        0.2,
-        Color::WHITE,
-    );
+        // Draw a circle just above the ground plane at that position.
+        gizmos.circle(
+            Isometry3d::new(
+                point + ground.up() * 0.01,
+                Quat::from_rotation_arc(Vec3::Z, ground.up().as_vec3()),
+            ),
+            0.2,
+            Color::WHITE,
+        );
+    }
 }
 
 #[derive(Component)]