Merge commit 'f450cc7b'

* commit 'f450cc7b':
  h264: eliminate decode_postinit()

Also includes fixes from 1f7b4f9a and e344e651.

Original patch replace H264Context.next_output_pic (H264Picture *) by
H264Context.output_frame (AVFrame *). This change is discarded as it
is incompatible with the frame reconstruction and motion vectors
display code which needs the extra information from the H264Picture.

Merged-by: Clément Bœsch <u@pkh.me>
Merged-by: Matthieu Bouron <matthieu.bouron@gmail.com>