Pass packets as const AVPacket& through the decode seam

[NicolasHug]

DeviceInterface::send_packet() only ever reads the packet - the default
implementation hands it to avcodec_send_packet(), which takes a const one.
Except that BetaCuda's bitstream filter path did not: av_bsf_send_packet()
takes ownership of what it is given and resets it, so sending a packet
through the seam could empty it, silently, with nothing in the signature
saying so.

SingleStreamDecoder never noticed, because its packet is dead after the
send. PacketDecoder did: the packet it is handed is a user-visible Python
object that has to survive being decoded, so it referenced it into a
throwaway packet before every single send - a per-packet allocation, on
every device, to defend against a contract that was never stated.

State the contract instead. The seam takes a const AVPacket&, and the one
implementation that needs ownership references the packet itself, where the
reason for it is local.

[pytorch-bot]

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