SOUT: implement drain

Currently, draining is done when deleting a sout stream.

Indeed, for the sout case, draining is not handled from vlc_input_decoder_Drain() but from vlc_input_decoder_Delete().

This makes the sout drain not interruptible at all since the input_thread_t already reached EOS and vlc_input_decoder_IsEmpty return true for the sout case.

Since we want to postpone any sout rework to VLC 5.0 (mostly, to make Send and the future Drain asynchronous).

I propose that we implement a synchronous drain in sout:

• We move draining from Del to the new Drain on sout stream that need it.
• We register an interrupt context in sout modules implementing Drain to make the Drain call interruptible.

This is ugly and far from perfect, but I don't see us doing major sout rework since we are already late for VLC 4.0.

rejected: The drain should be asynchronous with a drained callback.

This will make transcode and the future whisper sout !4468 (closed) interruptible when reaching the end of stream.