Ogg streams were invalid.
This patch ensures all streams have a way to end independently and
no other stream could start at arbitrary places. Also simplifies
a bit some code.

The ogg muxer suffers from an insufficient muxer cache value, and usually
starts streams before all streams have been declared.
The previous behaviour ( closing and restarting a new group of streams ) is kept
and fixed. But this is really not optimal.
We might need to consider increasing that cache for ogg.

It also handles correctly chained streams.