If not set, they will be defined using the channel layout setting, which is much more convenient when using swr_alloc() instead of swr_alloc_set_opts().
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com> Signed-off-by: Michael Niedermayer <michaelni@gmx.at>