New libav will not call the release_buffer callback anymore when
avcodec_default_get_buffer() is called from get_buffer. Releasing of the
memory in a picture should now be done by registering a callback to the
avbuffer objects in the picture. There is some compatibility code to
wrap the memory we provide in get_buffer in avbuffer with a callback to
release_buffer but that is not done when avcodec_default_get_buffer()
is called.

Work around this by adding a dummy avbuffer object to the picture that
will release the frame.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=721077