exr: support grayscale images

[ami_stuff]

C:\>ffmpeg -i C:\none_grayscale.exr
ffmpeg version N-80233-g27c1eae Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.5.0 (GCC) 20100414 (Fedora MinGW 4.5.0-1.fc14)
  configuration: --prefix=/var/www/users/research/ffmpeg/snapshots/build --arch=
x86 --target-os=mingw32 --cross-prefix=i686-pc-mingw32- --cc='ccache i686-pc-min
gw32-gcc' --enable-pthreads --enable-memalign-hack --enable-runtime-cpudetect --
enable-cross-compile --enable-static --disable-shared --extra-libs='-lws2_32 -lw
inmm -lpthread' --extra-cflags='--static -I/var/www/users/research/ffmpeg/snapsh
ots/build/include' --extra-ldflags='-static -L/var/www/users/research/ffmpeg/sna
pshots/build/lib' --enable-bzlib --enable-zlib --enable-gpl --enable-version3 --
enable-nonfree --enable-libx264 --enable-libspeex --enable-libtheora --enable-li
bvorbis --enable-libfaac --enable-libxvid --enable-libopencore-amrnb --enable-li
bopencore-amrwb --enable-libmp3lame --enable-libfreetype --enable-libvpx --disab
le-decoder=libvpx
  libavutil      55. 24.100 / 55. 24.100
  libavcodec     57. 45.100 / 57. 45.100
  libavformat    57. 37.101 / 57. 37.101
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 46.101 /  6. 46.101
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[exr @ 0x28dd880] Missing red channel.
[exr @ 0x28dd880] Missing blue channel.
[exr_pipe @ 0x28d3d20] decoding for stream 0 failed
[exr_pipe @ 0x28d3d20] Could not find codec parameters for stream 0 (Video: exr,
 none): unspecified size
Consider increasing the value for the 'analyzeduration' and 'probesize' options
Input #0, exr_pipe, from 'C:\none_grayscale.exr':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: exr, none, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

[Carl Eugen Hoyos]

Fixed by Martin Vignali in 7ebd13fc8b2d849a9a6f9f42b3c69f1aa254a4c6