qdrw: fix decoding of 4bpp (lead)

[ami_stuff]

this is what lead software creates, decodes ok with quicktime/imagemagick

quicktime says quickdraw, 256colors

​https://www.leadtools.com/help/leadtools/v19/vector/api/pctfmt.html

C:\>ffmpeg -i C:\lead_4bit.pct out.bmp
ffmpeg version 3.2.git Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --disable-pthreads --disable-ffprobe --enable-gpl
  libavutil      55. 47.100 / 55. 47.100
  libavcodec     57. 81.100 / 57. 81.100
  libavformat    57. 66.102 / 57. 66.102
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 74.100 /  6. 74.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
  libpostproc    54.  2.100 / 54.  2.100
[qdraw @ 0237d8e0] Warning: not compiled with thread support, using thread emula
tion
Input #0, qdraw_pipe, from 'C:\lead_4bit.pct':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: qdraw, pal8, 1024x768, 25 tbr, 25 tbn, 25 tbc
[qdraw @ 02763740] Warning: not compiled with thread support, using thread emula
tion
[bmp @ 02377d00] Warning: not compiled with thread support, using thread emulati
on
Output #0, image2, to 'out.bmp':
  Metadata:
    encoder         : Lavf57.66.102
    Stream #0:0: Video: bmp, pal8, 1024x768, q=2-31, 200 kb/s, 25 fps, 25 tbn, 2
5 tbc
    Metadata:
      encoder         : Lavc57.81.100 bmp
Stream mapping:
  Stream #0:0 -> #0:0 (qdraw (native) -> bmp (native))
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A speed=N/A
video:769kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing o
verhead: unknown