Support decoding transparency information in dnxhd streams

[Carl Eugen Hoyos]

​http://thread.gmane.org/gmane.comp.video.ffmpeg.libav.user/13205

A user uploaded a dnxhd sample that contains transparency information (the black background is supposed to be transparent), FFmpeg reports yuv422p10le.

$ ffmpeg -i Untitled_dnhd.mov
ffmpeg version N-63852-gf341340 Copyright (c) 2000-2014 the FFmpeg developers
  built on Jun  9 2014 16:59:00 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 89.100 / 52. 89.100
  libavcodec     55. 66.100 / 55. 66.100
  libavformat    55. 42.101 / 55. 42.101
  libavdevice    55. 13.101 / 55. 13.101
  libavfilter     4.  7.100 /  4.  7.100
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Untitled_dnhd.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 537199360
    compatible_brands: qt
    creation_time   : 2014-05-29 21:36:09
  Duration: 00:00:01.03, start: 0.000000, bitrate: 842317 kb/s
    Stream #0:0(eng): Video: dnxhd (AVdn / 0x6E645641), yuv422p10le, 1920x1080, 841432 kb/s, 29.97 fps, 29.97 tbr, 1000k tbn, 1000k tbc (default)
    Metadata:
      creation_time   : 2014-05-29 21:36:09
      handler_name    : Apple Alias Data Handler
      encoder         : DNxHD  709
At least one output file must be specified

[Carl Eugen Hoyos]

Sample uploaded to ​http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket3707/

[Elon Musk]

It looks like alpha is stored as raw separate 8bit plane.
Single sample is not enough.

[Carl Eugen Hoyos]

Additional, possibly very useful sample from forum user ils_fl uploaded.

[Carl Eugen Hoyos]

An additional sample is attached to ticket #8427