Function refine_subsampled_motion_estimate 

fn refine_subsampled_motion_estimate<T: Pixel>(
    fi: &FrameInvariants<T>,
    ts: &TileStateMut<'_, T>,
    w: usize,
    h: usize,
    tile_bo: TileBlockOffset,
    ref_frame: RefType,
    ssdec: u8,
    lambda: u32,
    bit_depth: usize,
    cpu_feature_level: CpuFeatureLevel,
) -> Option<MotionSearchResult>

Refine motion estimation that was computed one level of subsampling up.