h264: deMpegEncContextize

      Most of the changes are just trivial are just trivial replacements of
fields from MpegEncContext with equivalent fields in H264Context.
Everything in h264* other than h264.c are those trivial changes.

The nontrivial parts are:
1) extracting a simplified version of the frame management code from
   mpegvideo.c. We don't need last/next_picture anymore, since h264 uses
   its own more complex system already and those were set only to appease
   the mpegvideo parts.
2) some tables that need to be allocated/freed in appropriate places.
3) hwaccels -- mostly trivial replacements.
   for dxva, the draw_horiz_band() call is moved from
   ff_dxva2_common_end_frame() to per-codec end_frame() callbacks,
   because it's now different for h264 and MpegEncContext-based
   decoders.
4) svq3 -- it does not use h264 complex reference system, so I just
   added some very simplistic frame management instead and dropped the
   use of ff_h264_frame_start(). Because of this I also