H264 Support SP frames exactly

[Carl Eugen Hoyos]

Only the first nine frames of fate-suite/h264-conformance/sp2_bt_b.h264 are decoded correctly, the remaining frames show visual artefacts.

$ ffmpeg -i fate-suite/h264-conformance/sp2_bt_b.h264
ffmpeg version N-42404-gb4d0c3d Copyright (c) 2000-2012 the FFmpeg developers
  built on Jul 10 2012 09:31:25 with gcc 4.3.2
  configuration: --cc=/usr/local/gcc-4.3.2/bin/gcc --enable-gpl --enable-libopenjpeg --enable-libvorbis --enable-libspeex --enable-libmp3lame --enable-libtheora --extra-ldflags=-lm --enable-libvpx --enable-libxavs
  libavutil      51. 64.100 / 51. 64.100
  libavcodec     54. 33.100 / 54. 33.100
  libavformat    54. 15.104 / 54. 15.104
  libavdevice    54.  1.100 / 54.  1.100
  libavfilter     3.  2.100 /  3.  2.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
[h264 @ 0x90333c0] max_analyze_duration 5000000 reached at 5000000
[h264 @ 0x90333c0] Estimating duration from bitrate, this may be inaccurate
Input #0, h264, from 'fate-suite/h264-conformance/sp2_bt_b.h264':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (Extended), yuv420p, 176x144, 25 fps, 25 tbr, 1200k tbn, 50 tbc
At least one output file must be specified