Currently, when a demuxer thread is choked, it will avoid queuing more
packets, but any packets already present on the thread queue will still be
processed.
This can be quite wasteful if the choke is due to e.g. decoder not being
needed yet, such as in a filter graph involving concatenation-style filters.
Adding the ability to propagate the choke status to the thread queue directly
allows downstream decoders and filter graphs to avoid unnecessary work and
buffering.
Reduces the effective latency between scheduler updates and changes in the
thread workfload.
The queue needs to track each frame/packet's stream index, this is
achieved by maintaining a parallel AVFifo instance for that purpose.
This is simpler than implementing custom AVContainerFifo callbacks.
This ensures that tq_receive() will always return EOF after all streams
were receive-finished, even though the sending side might not have
closed them yet. This may allow the receiver to avoid manually tracking
which streams it has already closed.
