Directional light shadows are cut off

[PJB3005]

Shadows are cut off, and this cutoff is inconsistent between eyes.

To test, replace the light spawn in the 3d_scene example with a directional light:

    commands.spawn((
        DirectionalLight {
            illuminance: 16_000.0, // sunny.
            shadows_enabled: true,
            ..Default::default()
        },
        Transform::from_xyz(4.0, 8.0, 4.0).looking_at(Vec3::ZERO, Dir3::Y),
    ));