The altref frame is packed along with the next P frame. So that
outside of the codec there are now only two types of frames P and I.
Also, now it is one frame in and one frame out with respect to the
codec. Apart from that, all the frames are length encoded with the
length of each frame appended to the frame itself. There are
two categories of frames and each of them will look as follows:

  - Packed frames (an altref along with the succeeding p frame)
    - altref_frame_data | altref_lenngth | frame_data | length
  - Unpacked frames (all frames other than the above)
    - frame_data | length

Change-Id: If1eabf5c473f7d46b3f2d026bd30c803588c5330