This reverts commit 3c9681e1.
It breaks DMO/Qt decoding. It can be commited again when a fix
is found.

This removes a silly error message.

It will work most of the time.
Subtitle are not correctly handled.
It will be ignored if no video.

No default key is assigned.

It is ignored.

That was only an optimization, and it fails thread-safety.
In any case, a failed decoder will still dequeue (and discard) blocks.

And fix a potential memleak.