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This also fix the crash when clicking on play without any media loaded

Added some doc

This way, the TrackWorkflow can return a black output when nothing has
to be rendered

However, they must NOT be simultaneous (for now !)

Removed debug that I (obviously) merged badly in a previous commit

It now starts, load, and pause all by itself, without any intervention
of the TrackWorkflow, nor anything else.
Added some debug to detect a crash, the problem is probably inside of
the metadatamanager.

The aim is to launch a clip workflow, and let it start the playback, but
pause as the first frame.

This has only be tester with a Clip end date, not with an "unset" Clip
end.
Removed debug.

The Main/Tracks workflow remains really basic...

There's still a size/chroma/pitch/something_else problem, but this should work
soon :)

Corrected Clip workflow, seems kinda functionnal, but will need more
testing...

Removed useless stuff from VLCMedia