vc1 gray decoding produces artefacts

[Carl Eugen Hoyos]

The following produces ugly artefacts since e5c3b51cd1ba5167ac224c41b905928f1bd0eb8d

$ ffmpeg -flags +gray -i fate-suite/vc1/SA00040.vc1 -qscale 2 out.avi
ffmpeg version N-71990-gc97d30f Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl --enable-gray
  libavutil      54. 23.101 / 54. 23.101
  libavcodec     56. 38.100 / 56. 38.100
  libavformat    56. 32.100 / 56. 32.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 16.101 /  5. 16.101
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
[vc1 @ 0x2e5b380] Failed to open codec in av_find_stream_info
[NULL @ 0x2e5bda0] ignoring invalid SAR: 0/0
Input #0, vc1, from 'fate-suite/vc1/SA00040.vc1':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: vc1 (Advanced), yuv420p, 176x144 [SAR 1:1 DAR 11:9], 25 tbr, 1200k tbn, 25 tbc
Please use -q:a or -q:v, -qscale is ambiguous
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf56.32.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 176x144 [SAR 1:1 DAR 11:9], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.38.100 mpeg4
Stream mapping:
  Stream #0:0 -> #0:0 (vc1 (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
frame=   15 fps=0.0 q=2.0 Lsize=      92kB time=00:00:00.64 bitrate=1176.4kbits/s
video:86kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 6.992713%

Note that the input is shown as vc1 (Advanced), yuv420p although reading the source code, it should be vc1 (Advanced), gray.

[Michael Niedermayer]

Fixed in 982e7bbfa6b57c7173d3f774fbbb735aed3291f0
and 9025bc588ad441b9b0eccd69aa91caf5d2ac49ac