函数 swr_drop_output 

pub unsafe extern "C" fn swr_drop_output(
    s: *mut SwrContext,
    count: c_int,
) -> c_int

Drops the specified number of output samples.

This function, along with swr_inject_silence(), is called by swr_next_pts() if needed for “hard” compensation.

@param s allocated Swr context @param count number of samples to be dropped

@return >= 0 on success, or a negative AVERROR code on failure