函数 av_reduce 

pub unsafe extern "C" fn av_reduce(
    dst_num: *mut c_int,
    dst_den: *mut c_int,
    num: i64,
    den: i64,
    max: i64,
) -> c_int

Reduce a fraction.

This is useful for framerate calculations.

@param[out] dst_num Destination numerator @param[out] dst_den Destination denominator @param[in] num Source numerator @param[in] den Source denominator @param[in] max Maximum allowed values for dst_num & dst_den @return 1 if the operation is exact, 0 otherwise