Function service_copy_frames 

pub(crate) fn service_copy_frames(
    state: &mut AppState,
    node: &mut OutputNode,
    source: Option<&Dmabuf>,
    width: i32,
    height: i32,
    damage_rects: &[Rectangle<i32, Physical>],
)

Complete parked ext-image-copy-capture frames for this output from the frame the render pass just produced. A dmabuf target is filled by one GPU blit from the composited buffer (composited_target); an shm target by one readback (GLES) or one row copy (pixman). With no damage, a session that has already received content keeps its frame parked, so a static screen costs capture clients nothing.