Rename it to pf_process_output. The output buffer is now dequeued/parsed
outside of the pf callback.

This commit also removes the decoder_NewPicture HACK to know if decoder is
flushing.

Split put_in into dequeue_in and queue_in.
Split get_out into dequeue_out and get_out

When the thread is cancelled, it will be exited before processing all input
blocks.

This does not make any sense. Spaces are normally encoded as %20 (+ is for
query parameters values only).

Also fix variable shadowing.

Return an error code if the stream is not a directory (similar to
STREAM_GET_SIZE).

The demuxers that can this function predate directory/explicit playlist
support.