bmp 1bpp with palette decodes in negative

[ami_stuff]

(issue 1421)

C:\>ffmpeg -i Test_1bpp_gray.bmp
ffmpeg version N-34318-g8475ec1, Copyright (c) 2000-2011 the FFmpeg developers
  built on Oct 31 2011 17:50:05 with gcc 4.6.1
  configuration: --enable-gpl --enable-version3 --enable-runtime-cpudetect --ena
ble-avisynth --enable-bzlib --enable-frei0r --enable-libopencore-amrnb --enable-
libopencore-amrwb --enable-libfreetype --enable-libgsm --enable-libmp3lame --ena
ble-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --en
able-libtheora --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis
--enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib

  libavutil    51. 22. 0 / 51. 22. 0
  libavcodec   53. 26. 0 / 53. 26. 0
  libavformat  53. 18. 0 / 53. 18. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 45. 3 /  2. 45. 3
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, image2, from 'Test_1bpp_gray.bmp':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: bmp, monob, 1024x768, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

[cehoyos]

Should be fixed.