Function scale_plane 

fn scale_plane(
    src: &[u8],
    sw: usize,
    sh: usize,
    sstride: usize,
    dst: &mut [u8],
    dw: usize,
    dh: usize,
    tmp: &mut [Vec<u8>; 2],
)

Resample a plane into dst (tightly packed dw x dh), halving first while the source is more than twice the destination in either dimension.