sunras: support 4bpp

[ami_stuff]

decodes with irfanview

looks valid:

​http://www.leadtools.com/help/leadtools/v175/dh/to/leadtools.topics.leadtools~l.topics.fileformatssun.html

C:\>ffmpeg -i 4bpp.ras
ffmpeg version N-36492-g7f83db3, Copyright (c) 2000-2012 the FFmpeg developers
  built on Jan  5 2012 01:57:12 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. 34.100 / 51. 34.100
  libavcodec     53. 53.100 / 53. 53.100
  libavformat    53. 29.100 / 53. 29.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 57.101 /  2. 57.101
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  5.100 /  0.  5.100
  libpostproc    51.  2.100 / 51.  2.100
[sunrast @ 02645B60] invalid depth
[image2 @ 020E2320] Could not find codec parameters (Video: sunrast)
4bpp.ras: could not find codec parameters

[Carl Eugen Hoyos]

Could you test attached bad patch with width 1025?

[ami_stuff]

doesn't work correctly

[Carl Eugen Hoyos]

Both 1bpp_1025.ras and 4bpp_1025.ras work fine here on LE and BE with attached patch, what doesn't work for you?

[ami_stuff]

I applied the patch by hand (without git) and probably messed something. Yes it works fine.