Support gray mjpeg encoding

[Carl Eugen Hoyos]

The mjpeg encoder does not support gray output:

$ ffmpeg -i gray.jpg out.jpg
ffmpeg version N-67754-g30156ea Copyright (c) 2000-2014 the FFmpeg developers
  built on Nov 17 2014 11:54:23 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 13.100 / 54. 13.100
  libavcodec     56. 12.101 / 56. 12.101
  libavformat    56. 13.100 / 56. 13.100
  libavdevice    56.  3.100 / 56.  3.100
  libavfilter     5.  2.103 /  5.  2.103
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, image2, from 'gray.jpg':
  Duration: 00:00:00.04, start: 0.000000, bitrate: 3674 kb/s
    Stream #0:0: Video: mjpeg, gray(bt470bg/unknown/unknown), 256x256 [SAR 72:72 DAR 1:1], 25 tbr, 25 tbn, 25 tbc
[swscaler @ 0x242b400] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'out.jpg':
  Metadata:
    encoder         : Lavf56.13.100
    Stream #0:0: Video: mjpeg, yuvj444p(pc), 256x256 [SAR 1:1 DAR 1:1], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.12.101 mjpeg
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> mjpeg (native))
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=4.1 Lsize=N/A time=00:00:00.04 bitrate=N/A
video:12kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown