avcodec_parameters_to_context() overwrites codec_type and codec_id. But
we already set these by passing the selected AVCodec to
avcodec_alloc_context3(). It's entirely possible that at least codec_id
is different when forcing codecs with --ad/--vd. It's probably better
not to cause confusion by overwriting them. It might even trigger
undefined behavior in libavcodec (how it behaves or whether codec_id is
supposed to be strictly set is unknown, though).