Function compose_host_frame 

pub(crate) fn compose_host_frame(
    renderer: &mut GlesRenderer,
    overlay: Option<&OverlayState>,
    src: &Dmabuf,
    target: &mut Dmabuf,
    size: (i32, i32),
) -> Result<SyncPoint, String>

Re-compose src (the retained host frame) into target, with the watermark where overlay is given: the path a moving watermark needs, since re-drawing over the same retained buffer would leave trails.