libcmml: Multitrack input media in CMML

# Multitrack input media in CMML

With CMML it is possible to create markup for several, potentially interleaved, media files. This is specified in the stream tag by giving several source tags which each point to a media file. The "start" tag specifies at what time the media file should be inserted. In case of temporally overlapping media files they are regarded as interleaved files.

This is an example of such a specification:

    <stream basetime="npt:0">
      <import contenttype="video/x-theora" src="fish.ogg" start="npt:0"/>
      <import contenttype="audio/x-vorbis" src="fishsound.ogg" start="npt:0"/>
      <import contenttype="audio/x-speex" src="comments.spx" start="npt:7"/>
    </stream>