Function detach_retained_gpu 

pub(crate) fn detach_retained_gpu(
    renderer: Option<&mut GlesRenderer>,
    gbm: Option<&Device<File>>,
    host: &HostSession,
    cap: &mut WlCapture,
    display_id: u32,
    size: (i32, i32),
)

HostSession::detach_retained for a GPU frame, which only a zero-copy session takes: the renderer copies it into a buffer of the capture’s own, which re-encodes then read, and the stream’s buffer goes back once no frame in flight reads it.