Function prepare_layer_backdrops 

pub fn prepare_layer_backdrops(
    extracted: Res<'_, ExtractedUiLayers>,
    store: ResMut<'_, LayerTextureStore>,
    filter_pipeline: Option<Res<'_, LayerFilterPipeline>>,
    blit_pipeline: Option<Res<'_, BackdropBlitPipeline>>,
    specialized_filters: ResMut<'_, SpecializedRenderPipelines<LayerFilterPipeline>>,
    specialized_blits: ResMut<'_, SpecializedRenderPipelines<BackdropBlitPipeline>>,
    pipeline_cache: Res<'_, PipelineCache>,
    render_device: Res<'_, RenderDevice>,
    render_queue: Res<'_, RenderQueue>,
    cameras: Query<'_, '_, &ExtractedCamera>,
    time: Res<'_, Time>,
    meta: ResMut<'_, BackdropMeta>,
)

Stage every backdrop’s blit + filter run for this frame. Mirrors prepare_layer_filters’ three phases (stage → write buffers + bind groups → predict validity), with two deliberate differences: staging is unconditional per frame (the source frame is live), and the validity prediction requires the BLIT pipeline compiled too — the chain would otherwise filter a garbage snapshot.