Currently uncoded frames (i.e. packets whose data actually points to an
AVFrame) are not refcounted. As a consequence, calling av_packet_unref()
on them will not free them, but may simply make sure that they leak by
losing the pointer to the frame.

This commit changes this by actually making uncoded frames refcounted.
In order not to rely on sizeof(AVFrame) (which is not part of the public
API and so must not be used here in libavformat) the packet's data is
changed to a (padded) buffer containing just a pointer to an AVFrame.
Said buffer is owned by an AVBuffer with a custom free function that
frees the frame as well as the buffer. Thereby the pointer/the AVBuffer
owns the AVFrame.

Said ownership can actually be transferred by copying and resetting
the pointer, as might happen when actually writing the uncoded frames
in AVOutputFormat.write_uncoded_frame() (although currently no muxer
makes use of this possibility).

This makes packets containing uncoded frames compatible with
av_packet_unref(). This already has three advantages in interleaved mode:
1. If an error happens at the preparatory steps (before the packet is
put into the interleavement queue), the frame is properly freed.
2. If the trailer is never written, the frames still in the
interleavement queue will now be properly freed by
ff_packet_list_free().
3. The custom code for moving the packet to the packet list in
ff_interleave_add_packet() can be removed.

It will also simplify fixing further memleaks in future commits.
Suggested-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>