add support for 64bit PNG

[ami_stuff]

C:\>ffmpeg -i 64bit.png
ffmpeg version N-34549-g13b7781, Copyright (c) 2000-2011 the FFmpeg developers
  built on Nov  6 2011 22:02:08 with gcc 4.6.1
  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. 24. 0 / 51. 24. 0
  libavcodec   53. 28. 0 / 53. 28. 0
  libavformat  53. 19. 0 / 53. 19. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 47. 0 /  2. 47. 0
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[image2 @ 020A2320] Could not find codec parameters (Video: png, 1023x767)
64bit.png: could not find codec parameters

[Elon Musk]

As you can see there is neither input nor output for 64bit variants so this wish depends on another wish.
..... rgba64be 4 64
..... rgba64le 4 64
..... bgra64be 4 64
..... bgra64le 4 64

[Carl Eugen Hoyos]

Decoding 64bit png works now, the images can be reencoded to dpx, pam and sgi (and png)

For the missing colourspace transformation, please follow ticket #503.