Download of "double" videos on Opencast

[OsziBatyam]

In some Lectures "double" videos are used where one is a video of the professor talking and the other is the presentation (they play then together side-by-side or with one on the corner of the other one. ) Currently it is only downloading of these videos, seemingly random which one.

[D-VR]

This is probably related to the changes which broke syncmymoodle in #125, seems like an OpenCast episode can have multiple multimedias attached, and we should probably handle this.

Different approaches:

• Naive: just download all videos and throw them in a folder
• maybe there is information on how the videos are combined, extract that and re-encode (problematic for >1 hour course videos)
• Bundle a local viewer for the OpenCast episode

Not straightforward to do imo, open for further discussion or PR's on this issue