Only enable mouse movement when pressed in scene_viewer (#4405)

# Objective

- Only move the camera when explicitly wanted, otherwise the camera goes crazy if the cursor isn't already in the middle of the window when it opens.

## Solution

- Check if the Left mouse button is pressed before updating the mouse delta
- Input is configurable

examples/tools/scene_viewer.rs

@@ -18,6 +18,8 @@ fn main() {
     println!(
         "
 Controls:
+    MOUSE  - Move camera orientation
+    LClick - Enable mouse movement
     WSAD   - forward/back/strafe left/right
     LShift - 'run'
     E      - up
@@ -356,6 +358,7 @@ struct CameraController {
     pub key_up: KeyCode,
     pub key_down: KeyCode,
     pub key_run: KeyCode,
+    pub key_enable_mouse: MouseButton,
     pub walk_speed: f32,
     pub run_speed: f32,
     pub friction: f32,
@@ -377,6 +380,7 @@ impl Default for CameraController {
             key_up: KeyCode::E,
             key_down: KeyCode::Q,
             key_run: KeyCode::LShift,
+            key_enable_mouse: MouseButton::Left,
             walk_speed: 5.0,
             run_speed: 15.0,
             friction: 0.5,
@@ -390,17 +394,12 @@ impl Default for CameraController {
 fn camera_controller(
     time: Res<Time>,
     mut mouse_events: EventReader<MouseMotion>,
+    mouse_button_input: Res<Input<MouseButton>>,
     key_input: Res<Input<KeyCode>>,
     mut query: Query<(&mut Transform, &mut CameraController), With<Camera>>,
 ) {
     let dt = time.delta_seconds();
 
-    // Handle mouse input
-    let mut mouse_delta = Vec2::ZERO;
-    for mouse_event in mouse_events.iter() {
-        mouse_delta += mouse_event.delta;
-    }
-
     if let Ok((mut transform, mut options)) = query.get_single_mut() {
         if !options.initialized {
             let (_roll, yaw, pitch) = transform.rotation.to_euler(EulerRot::ZYX);
@@ -454,6 +453,14 @@ fn camera_controller(
             + options.velocity.y * dt * Vec3::Y
             + options.velocity.z * dt * forward;
 
+        // Handle mouse input
+        let mut mouse_delta = Vec2::ZERO;
+        if mouse_button_input.pressed(options.key_enable_mouse) {
+            for mouse_event in mouse_events.iter() {
+                mouse_delta += mouse_event.delta;
+            }
+        }
+
         if mouse_delta != Vec2::ZERO {
             // Apply look update
             let (pitch, yaw) = (