函数 av_channel_layout_index_from_channel 

pub unsafe extern "C" fn av_channel_layout_index_from_channel(
    channel_layout: *const AVChannelLayout,
    channel: AVChannel,
) -> c_int

Get the index of a given channel in a channel layout. In case multiple channels are found, only the first match will be returned.

@param channel_layout input channel layout @param channel the channel whose index to obtain @return index of channel in channel_layout on success or a negative number if channel is not present in channel_layout.