AV_DISPOSITION_FORCED not set for mov_text

[Carl Eugen Hoyos]

(videolan ticket 16803)
A user provided a mov sample that contains forced subtitles, the relevant flag does not get set by FFmpeg.

$ ffmpeg -i example.mp4
ffmpeg version N-79716-g0ff3489 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl --enable-libwebp --enable-libvpx
  libavutil      55. 23.100 / 55. 23.100
  libavcodec     57. 38.100 / 57. 38.100
  libavformat    57. 35.100 / 57. 35.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 44.100 /  6. 44.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x3977280] Invalid SampleDelta -10000 in STTS, at 4 st:2
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'example.mp4':
  Metadata:
    major_brand     : iso4
    minor_version   : 1
    compatible_brands: iso4hvc1iso6
    creation_time   : 2016-04-12 05:37:44
    encoder         : MP4Box-0.6.2-DEV-rev228-x64
  Duration: 1193:04:27.30, start: 0.000000, bitrate: 0 kb/s
    Stream #0:0(und): Video: hevc (Main) (hvc1 / 0x31637668), yuv420p(tv), 1920x1080 [SAR 1:1 DAR 16:9], 4885 kb/s, 25 fps, 25 tbr, 25 tbn (default)
    Metadata:
      creation_time   : 2016-04-12 05:37:44
      handler_name    : H.265 Videospur
    Stream #0:1(deu): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 96 kb/s (default)
    Metadata:
      creation_time   : 2016-03-14 19:22:53
      handler_name    : deutsche Tonspur
    Stream #0:2(deu): Subtitle: mov_text (tx3g / 0x67337874), 1920x128, 0 kb/s (default)
    Metadata:
      rotate          : 0
      creation_time   : 2016-04-12 05:37:46
      handler_name    : deutscher Untertitel (erzwungen)
At least one output file must be specified