Clean up background motion vectors when prepass is disabled

[issam3105]

Objective

• Fix a render validation error when TemporalAntiAliasing is removed from a camera while DepthPrepass remains enabled.
• The background motion vectors pipeline state could remain on the render-world view after MotionVectorPrepass was removed, causing the prepass node to try to render background motion vectors without a motion vector target.

Solution

Add a cleanup system for background motion vector state.

Testing

Press Space to disable TAA while keeping DepthPrepass on the camera. Before this PR, it crash with:

Render pipeline targets are incompatible with render pass
Incompatible color attachments at indices []: the RenderPass uses textures with formats [] but the RenderPipeline with 'background_motion_vectors_pipeline' label uses attachments with formats [None, Some(Rg16Float), None, None]

Minimal repro example:

use bevy::anti_alias::taa::TemporalAntiAliasing;
use bevy::core_pipeline::prepass::{DepthPrepass, MotionVectorPrepass};
use bevy::prelude::*;
use bevy::render::camera::{MipBias, TemporalJitter};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, toggle_taa)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn((
        Camera3d::default(),
        Msaa::Off,
        DepthPrepass,
        TemporalAntiAliasing::default(),
        Transform::from_xyz(0.0, 0.0, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
}

fn toggle_taa(
    keys: Res<ButtonInput<KeyCode>>,
    camera: Single<Entity, With<Camera3d>>,
    mut commands: Commands,
) {
    if keys.just_pressed(KeyCode::Space) {
        commands.entity(*camera).remove::<(
            TemporalAntiAliasing,
            TemporalJitter,
            MipBias,
            MotionVectorPrepass,
        )>();
    }
}