Support gray16a png

[ami_stuff]

the files comes from png testsuite

no gdb log, because I would need to recompile everything again to have debug build of ffplay (40 mins. wasted :()

C:\>ffplay basi4a16.png
ffplay version N-34704-g985e768, Copyright (c) 2003-2011 the FFmpeg developers
  built on Nov 11 2011 01:49:15 with gcc 4.6.2
  configuration: --disable-static --enable-shared --enable-gpl --enable-version3
 --disable-w32threads --enable-runtime-cpudetect --enable-avisynth --enable-bzli
b --enable-frei0r --enable-libopencore-amrnb --enable-libopencore-amrwb --enable
-libfreetype --enable-libgsm --enable-libmp3lame --enable-libopenjpeg --enable-l
ibrtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-li
bvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-l
ibx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil    51. 24. 1 / 51. 24. 1
  libavcodec   53. 31. 0 / 53. 31. 0
  libavformat  53. 20. 0 / 53. 20. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 47. 2 /  2. 47. 2
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, image2, from 'basi4a16.png':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: png, gray8a, 32x32, 25 tbr, 25 tbn, 25 tbc
[buffersink @ 00A72780] auto-inserting filter 'auto-inserted scale 0' between th
e filter 'src' and the filter 'out'
[scale @ 00A72A00] w:32 h:32 fmt:gray8a -> w:32 h:32 fmt:yuv420p flags:0x4

[cehoyos]

The sample is actually gray16a which is currently not supported by FFmpeg.

[cehoyos]

Crash should be fixed, the file can be decoded once gray16a will be supported.