Function alloc_cpu_slot 

fn alloc_cpu_slot(
    shm: &WlShm,
    qh: &QueueHandle<CaptureState>,
    format: u32,
    w: i32,
    h: i32,
    stride: i32,
) -> Result<SlotBuffer, String>

An shm-backed capture slot in a memfd pool the CPU consumer reads in place.