Subtitles (ssa/vobsub) not showing on matroska video with playback-test

Submitted by Arnaud Vrac @rawoul

Link to original bug (#687625)

Description

With 1.0.2 or git master, playback-test never shows any subtitles for the following video:

playback-test 0 http://absolut.zogzog.org/share/samples/mkv/paprika.mkv

The video contains 4 subtitle tracks, 1 SSA and 3 VOBSUB. MPlayer, VLC or Gstreamer cannot display the SSA track for some reason, however only Gstreamer fails to display the DVD subtitles. When switching to a DVD subtitle track, the following warning appears and the video freezes:

pre-colorspace: not negotiated (gstbasetransform.c(2081): gst_base_transform_handle_buffer (): /GstPlayBin:playbin0/GstPlaySink:playsink/GstBin:tbin/GstSubtitleOverlay:suboverlay/GstVideoConvert:pre-colorspace:
not negotiated)