psd: support 1bpp (bitmap) mode

[ami_stuff]

decodes ok with imagemagick

C:\>ffmpeg -i C:\8bits_per_ch_bitmap.psd
ffmpeg version 3.2.git Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --disable-pthreads --enable-gpl
  libavutil      55. 43.100 / 55. 43.100
  libavcodec     57. 68.100 / 57. 68.100
  libavformat    57. 61.100 / 57. 61.100
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 68.100 /  6. 68.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
  libpostproc    54.  2.100 / 54.  2.100
[psd @ 0224d660] Warning: not compiled with thread support, using thread emulati
on
[psd @ 0224d660] color mode 0 is not implemented. Update your FFmpeg version to
the newest one from Git. If the problem still occurs, it means that your file ha
s a feature which has not been implemented.
[psd_pipe @ 02243be0] decoding for stream 0 failed
[psd_pipe @ 02243be0] Could not find codec parameters for stream 0 (Video: psd,
none, 1023x767): unspecified pixel format
Consider increasing the value for the 'analyzeduration' and 'probesize' options
Input #0, psd_pipe, from 'C:\8bits_per_ch_bitmap.psd':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: psd, none, 1023x767, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

[Carl Eugen Hoyos]

Fixed by Martin Vignali in 658e626cc0d5de2d663f0e69c8836a87f51dd39f