sunras: support 1bpp with palette

[ami_stuff]

C:\>ffmpeg -i 1bpp.ras out.bmp
ffmpeg version N-36193-gf514695, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec 26 2011 17:50:37 with gcc 4.6.2
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libope
ncore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --en
able-libmp3lame --enable-libopenjpeg --enable-librtmp --enable-libschroedinger -
-enable-libspeex --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwben
c --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-
libxvid --enable-zlib
  libavutil      51. 33.100 / 51. 33.100
  libavcodec     53. 48.100 / 53. 48.100
  libavformat    53. 28.100 / 53. 28.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 54.100 /  2. 54.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  5.100 /  0.  5.100
  libpostproc    51.  2.100 / 51.  2.100
[sunrast @ 02645B60] useless colormap found or file is corrupted, trying to reco
ver
Input #0, image2, from '1bpp.ras':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: sunrast, monow, 1024x768, 25 tbr, 25 tbn, 25 tbc
Incompatible pixel format 'monow' for codec 'bmp', auto-selecting format 'monob'

[buffer @ 020F1340] w:1024 h:768 pixfmt:monow tb:1/1000000 sar:0/1 sws_param:
[buffersink @ 020F15C0] auto-inserting filter 'auto-inserted scale 0' between th
e filter 'src' and the filter 'out'
[scale @ 020F1A00] w:1024 h:768 fmt:monow -> w:1024 h:768 fmt:monob flags:0x4
Output #0, image2, to 'out.bmp':
  Metadata:
    encoder         : Lavf53.28.100
    Stream #0:0: Video: bmp, monob, 1024x768, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (sunrast -> bmp)
Press [q] to stop, [?] for help
[sunrast @ 02645B60] useless colormap found or file is corrupted, trying to reco
ver
frame=    1 fps=  0 q=0.0 Lsize=       0kB time=00:00:00.04 bitrate=   0.0kbits/
s
video:96kB audio:0kB global headers:0kB muxing overhead -100.000000%

[cehoyos]

Suboptimal patch attached.