[demuxer] Avoid memcpy on every demuxer packet

Avoids an unnecessary memcpy on every demuxer packet which for
high bitrate videos can be significant.
I don't understand that. The demuxer queue will contain exactly the same amount of data, it will just be pointing to original packets, and not to the copied packets.
This change has been in newclock4 test builds for the last week without issue, so I'd be surprised if there is a fundamental problem with the scheme.