Add support for AVI RAW (Y8)

[ami_stuff]

Attached patch adds Y8 FourCC.

The sample file comes from:

​https://trac.videolan.org/vlc/ticket/4985

C:\>ffmpeg -i "Luminance only (Y8, 16-235).avi"
ffmpeg version N-30956-g81ef892, Copyright (c) 2000-2011 the FFmpeg developers
  built on Jun 22 2011 17:34:15 with gcc 4.5.3
  configuration: --enable-gpl --enable-version3 --enable-memalign-hack --enable-
runtime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libo
pencore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --
enable-libmp3lame --enable-libopenjpeg --enable-librtmp --enable-libschroedinger
 --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libvpx --enabl
e-libx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil    51.  9. 1 / 51.  9. 1
  libavcodec   53.  7. 0 / 53.  7. 0
  libavformat  53.  4. 0 / 53.  4. 0
  libavdevice  53.  1. 1 / 53.  1. 1
  libavfilter   2. 23. 0 /  2. 23. 0
  libswscale    2.  0. 0 /  2.  0. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[avi @ 01E5A600] Could not find codec parameters (Video: Y8   / 0x20203859, 359x239)
Luminance only (Y8, 16-235).avi: could not find codec parameters

[ami_stuff]

Not realted to this issue, but there is

{ PIX_FMT_GRAY8, MKTAG(' ', ' ', 'Y', '8') },

in "libavcodec/raw.c"

so shouldn't

+ { CODEC_ID_RAWVIDEO, MKTAG(' ', ' ', 'Y', '8') },

be added to "libavformat/riff.c" as well?

[Carl Eugen Hoyos]

Pushed, thank you!