vf_yadif: factorize initializing the filtering callbacks

Do it all in config_props().
vf_yadif: fix out-of line reads

Some changes in the border pixels, visually indistinguishable.
get_buffer(): do not initialize the data.

There may be more decoders that rely on this. Those should be found and
fixed.
h264: simplify calls to ff_er_add_slice().

partitioned_frame is never set for h264 (as easily seen from git grep).
h264: remove a pointless if()

!encoding is always true, we do not have a H.264 encoder
svq3: remove a pointless if()

The H264 context is always uninitialized at this point.
error_resilience: decouple ER from MpegEncContext
mpegvideo: split ff_draw_horiz_band().

Split out dependency on MpegEncContext.
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 had to move some
   initialization code to svq3.

Additional fixes for chroma format and bit depth changes by
Janne Grunau <janne-libav@jannau.net>

Signed-off-by: Anton Khirnov <anton@khirnov.net>