now working for version 0.2
avfsrc: throw source_end when closing source
transfer set_output commands
change the way queue sink works

On releasing, queue sink remains alive until all urefs buffered in the sink
are sent. This is particularly helpful for the flow end, which was previously
dropped if the queue is full. There is also a new control command to flush
the buffered urefs.
simplify glxplay, using source_end on queues