Function decode_mb_p 

fn decode_mb_p(
    r: &mut BitReader<'_>,
    st: &mut DecState,
    mb_x: u32,
    mb_y: u32,
    qp: &mut i32,
    pps: &Pps,
    refr: &DecodedYuv,
) -> Result<(), &'static str>

インター: mb_type 0=16x16, 1=16x8, 2=8x16, 3/4=P_8x8。