Fix lighting example following emissive material changes in #13350 (#…

…13480)

# Objective

After the emissive material changes in #13350, the red and green point
lights in the `lighting` example turned white.

## Solution

This PR gives the point lights the `emissive_exposure_weight` property
in order for them to appear with correct color again.

## Testing

The `lighting` example before this fix:


![image](https://github.com/bevyengine/bevy/assets/143610747/be31d422-f616-4651-ab63-18ddfdba3773)

After this fix (looks the same as before #13350):


![image](https://github.com/bevyengine/bevy/assets/143610747/e5b5eab3-0588-4f30-bf74-2b52db7345ad)

examples/3d/lighting.rs

@@ -146,7 +146,7 @@ fn setup(
                 mesh: meshes.add(Sphere::new(0.1).mesh().uv(32, 18)),
                 material: materials.add(StandardMaterial {
                     base_color: RED.into(),
-                    emissive: Color::linear_rgba(713.0, 0.0, 0.0, 0.0),
+                    emissive: Color::linear_rgba(4.0, 0.0, 0.0, 0.0),
                     ..default()
                 }),
                 ..default()
@@ -174,7 +174,7 @@ fn setup(
                 mesh: meshes.add(Capsule3d::new(0.1, 0.125)),
                 material: materials.add(StandardMaterial {
                     base_color: LIME.into(),
-                    emissive: Color::linear_rgba(0.0, 713.0, 0.0, 0.0),
+                    emissive: Color::linear_rgba(0.0, 4.0, 0.0, 0.0),
                     ..default()
                 }),
                 ..default()