Function encode_shm_rows 

pub(crate) fn encode_shm_rows<T>(
    buffer: &Buffer,
    width: i32,
    height: i32,
    encode: impl FnOnce(&[u8]) -> Result<T, String>,
) -> Result<T, String>

Run encode on the rows of a client’s shm buffer a frame names (WlFrame::shm), height rows of width pixels packed four bytes each: the pool’s mapping is valid only inside this call, so the encode runs inside it. SHM_GONE where the pool was resized or destroyed under the frame.