Function copy_rows_into_shm 

pub(crate) fn copy_rows_into_shm(
    buffer: &WlBuffer,
    src: &[u8],
    width: i32,
    height: i32,
) -> Result<(), String>

Copy full rows from src (tight width*4 stride) into an Argb8888/Xrgb8888 shm buffer, honoring the destination’s own offset and stride.