Function run_backdrop_passes 

pub fn run_backdrop_passes(
    idx: usize,
    meta: &BackdropMeta,
    blit_pipeline: Option<&BackdropBlitPipeline>,
    main_texture: &TextureView,
    pipeline_cache: &PipelineCache,
    ctx: &mut RenderContext<'_, '_>,
)

Execute one layer’s staged backdrop work inside the capture-pass encoder: blit the frame region into the snapshot, then replay the chain snapshot → ping-pong. Called per layer from ui_layer_capture_pass, before stock ui_pass consumes the quads.