[demuxer] Avoid memcpy on every demuxer packet

Avoids an unnecessary memcpy on every demuxer packet which for
high bitrate videos can be significant.
ok, I see what you mean but this needs to be implemented differently. you can't construct an av packets with new operator and pray that it copies internals correctly. Instead of having a single packet as member, you want the demuxer to create a new packet on every read and give this to dvdplayer.