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This patch adds the ability to generate WebM DASH manifest XML using
ffmpeg. A sample command line would be as follows:

ffmpeg \
  -f webm_dash_manifest -i video1.webm \
  -f webm_dash_manifest -i video2.webm \
  -f webm_dash_manifest -i audio1.webm \
  -f webm_dash_manifest -i audio2.webm \
  -map 0 -map 1 -map 2 -map 3 \
  -c copy \
  -f webm_dash_manifest \
  -adaptation_sets “id=0,streams=0,1 id=1,streams=2,3” \
  manifest.xml

It works by exporting necessary fields as metadata tags in matroskadec
and use those values to write the appropriate XML fields as per the WebM
DASH Specification [1]. Some ideas are adopted from webm-tools project
[2].

[1]
https://sites.google.com/a/webmproject.org/wiki/adaptive-streaming/webm-dash-specification
[2]
https://chromium.googlesource.com/webm/webm-tools/+/master/webm_dash_manifest/

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>