This commit changes mpeg2_metadata to (a) use ff_bsf_get_packet_ref
instead of ff_bsf_get_packet (thereby avoiding one malloc and free per
filtered packet) and (b) to use only one packet structure at all,
thereby avoiding a call to av_packet_copy_props.

(b) has been made possible by the recent changes to ff_cbs_write_packet.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>