OutputContainer.mux() puts all packets with stream_index=0

Created by: vbkaisetsu

I tried to write a small encoder using the latest git repository for a movie that contains a video and an audio. However, both kinds of packets are mixed in to stream index "0".

OutputContainer.mux_one refers packet.struct.stream_index, but it does not set in Stream.encode. It may be removed on ae980126.