Subtitles from external webvtt files are ignored for on-demand profile

[matmoi]

Example: mp4dash --verbose --subtitles video.mp4 [+format=webvtt,+language=eng]subs.vtt

Subtitle files are simply dropped in mp4-dash.py

Proposed fix:
Source/Python/utils/mp4-dash.py | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Source/Python/utils/mp4-dash.py b/Source/Python/utils/mp4-dash.py
index 18df29d..3b39b5d 100755
--- a/Source/Python/utils/mp4-dash.py
+++ b/Source/Python/utils/mp4-dash.py
@@ -1389,7 +1389,7 @@ def main():
# for on-demand, we need to first extract tracks into individual media files
if options.on_demand:
(audio_sets, video_sets, subtitles_sets, mp4_files) = SelectTracks(options, media_sources)

•    media_sources = []
  

•    media_sources = filter(lambda x: x.format == "webvtt", media_sources) #Keep subtitles
     for track in sum(audio_sets.values()+video_sets.values(), []):
         print 'Extracting track', track.id, 'from', GetMappedFileName(track.parent.media_source.filename)
         track_file = tempfile.NamedTemporaryFile(dir = options.output_dir, delete=False)