In the common case that the input packet was already refcounted,
ff_interleave_add_packet would allocate a new AVPacketList, use
av_packet_ref to create a new reference to the buffer for the
AVPacketList's packet, interleave the packet and finally unreference
the original input packet.
This commit changes this: It uses av_packet_move_ref to transfer
the packet to its destination. In case the input packet is refcounted,
this saves an allocation and a free (of an AVBufferRef); if not, the
packet is made refcounted before moving it. When the input packet has
side data, one saves even more than one allocation+free.

Furthermore, when the packet is in reality an uncoded frame, a hacky
ad-hoc variant of av_packet_move_ref has been employed. Not any more.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>