Up until now, ff_cbs_write_packet always initialized the packet
structure it received without documenting this behaviour; furthermore,
the packet's buffer would (on success) be overwritten with the new
buffer without unreferencing the old. This meant that the input packet
had to be either clean (otherwise there would be memleaks) in which case
the initialization is redundant or uninitialized. ff_cbs_write_packet
was never used with uninitialized packets, so the initialization was
redundant. Worse yet, it forced callers to use more than one packet and
made it difficult to add side-data to a packet designated for output,
because said side-data could only be attached after the call to
ff_cbs_write_packet.

This has been changed. It is now allowed to use a non-blank packet.
The currently existing buffer will be unreferenced and replaced by
the new one, as will be the accompanying fields (i.e. data and size).
The rest isn't touched at all.

This change will enable us to use only one packet in the bitstream
filters that rely on CBS.

This commit also updates the documentation of ff_cbs_write_extradata
and ff_cbs_write_packet (to better describe existing behaviour and in
the latter case to also describe the new behaviour).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>