add 2bpp support in png decoder

[ami_stuff]

here is an 2bpp qt png sample

C:\>ffmpeg -i C:\2bpp.mov
ffmpeg version N-32290-g6fc1299, Copyright (c) 2000-2011 the FFmpeg developers
  built on Sep  2 2011 00:40:00 with gcc 4.6.1
  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. 14. 0 / 51. 14. 0
  libavcodec   53. 12. 0 / 53. 12. 0
  libavformat  53. 10. 0 / 53. 10. 0
  libavdevice  53.  3. 0 / 53.  3. 0
  libavfilter   2. 37. 0 /  2. 37. 0
  libswscale    2.  0. 0 /  2.  0. 0
  libpostproc  51.  2. 0 / 51.  2. 0

Seems stream 0 codec frame rate differs from container frame rate: 600.00 (600/1
) -> 23.98 (24000/1001)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'C:\2bpp.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 537199360
    compatible_brands: qt
    creation_time   : 2011-09-06 13:24:56
  Duration: 00:00:05.95, start: 0.000000, bitrate: 822 kb/s
    Stream #0.0(eng): Video: png (png  / 0x20676E70), pal8, 192x128, 820 kb/s, 2
4 fps, 23.98 tbr, 600 tbn, 600 tbc
    Metadata:
      creation_time   : 2011-09-06 13:24:56
At least one output file must be specified

[Michael Niedermayer]

Locally fixed, will push soon