函数 av_channel_layout_channel_from_index 

pub unsafe extern "C" fn av_channel_layout_channel_from_index(
    channel_layout: *const AVChannelLayout,
    idx: c_uint,
) -> AVChannel

Get the channel with the given index in a channel layout.

@param channel_layout input channel layout @param idx index of the channel @return channel with the index idx in channel_layout on success or AV_CHAN_NONE on failure (if idx is not valid or the channel order is unspecified)