Teletext subtitles are not automatically scaled

[Carl Eugen Hoyos]

​http://streams.videolan.org/issues/10919/
The subtitles on page 888 and page 889 are shown scaled with FFplay:

$ ffplay vlc-record-2014-03-13-03h01m41s-Arte.ts -txt_page 888

FFmpeg does not scale the subtitles:

$ ffmpeg -txt_page 888 -i vlc-record-2014-03-13-03h01m41s-Arte.ts -filter_complex [0:2][0:0]overlay -qscale 2 -t 50 out.avi
ffmpeg version N-74483-gb807f7e Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-libzvbi --enable-gpl
  libavutil      54. 30.100 / 54. 30.100
  libavcodec     56. 57.100 / 56. 57.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 34.100 /  5. 34.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
[mpegts @ 0x2101560] PES packet size mismatch
Input #0, mpegts, from 'vlc-record-2014-03-13-03h01m41s-Arte.ts':
  Duration: 00:00:51.58, start: 40205.850389, bitrate: 1772 kb/s
  Program 1
    Stream #0:0[0x44](fra,fra): Subtitle: dvb_teletext ([6][0][0][0] / 0x0006), 492x250
    Stream #0:1[0x45](???): Audio: mp2 ([3][0][0][0] / 0x0003), 48000 Hz, stereo, s16p, 128 kb/s
    Stream #0:2[0x46]: Video: h264 (Main) ([27][0][0][0] / 0x001B), yuv420p(tv, bt470bg), 720x576 [SAR 16:11 DAR 20:11], 25 fps, 25 tbr, 90k tbn, 50 tbc
Please use -q:a or -q:v, -qscale is ambiguous
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf56.40.101
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 720x576 [SAR 16:11 DAR 20:11], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc (default)
    Metadata:
      encoder         : Lavc56.57.100 mpeg4
    Stream #0:1(???): Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, stereo, fltp, 192 kb/s
    Metadata:
      encoder         : Lavc56.57.100 ac3
Stream mapping:
  Stream #0:0 (libzvbi_teletextdec) -> overlay:overlay (graph 0)
  Stream #0:2 (h264) -> overlay:main (graph 0)
  overlay (graph 0) -> Stream #0:0 (mpeg4)
  Stream #0:1 -> #0:1 (mp2 (native) -> ac3 (native))
Press [q] to stop, [?] for help
frame= 1238 fps=492 q=2.0 Lsize=   11230kB time=00:00:50.01 bitrate=1839.4kbits/s
video:9988kB audio:1166kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.684026%

The following works as expected:

$ ffmpeg -txt_page 888 -i vlc-record-2014-03-13-03h01m41s-Arte.ts -filter_complex [0:0]scale=pal,[0:2]overlay -qscale 2 -t 50 out.avi