Fix memory leak.

libsoup's asynchronous API is not thread safe.  By moving the calls
into the I/O thread, several crash bugs will be fixed.

Add the "complete" attribute, and set it in _session_callback().
_close() waits for it to become true.

Add attribute postponed_error, pass this GError to the caller.

Fix deadlock.

We'll add proper locking now, and there's no need to carry this
duplicate flag.

To report failures to callers who don't use _buffer().

Avoid recursive deadlock in _session_callback().

This was useless, because input_stream_read() blocks until data is
available.  Calling it in a loop doesn't make sense.

Pausing and resuming after every little chunk adds lots of overhead.
Add a lower level for resuming the stream.

curl_easy_pause(CURLPAUSE_CONT) may have added enough data to go into
pause again.

Call _update_fds() directly.  This is possible because it's only
called from within the I/O thread.

Fix deadlock.

Add -DNDEBUG to AM_CPPFLAGS.

Conflicts:
	NEWS
	configure.ac

Let the codec start with fresh buffers.  This should fix the remaining
seeking issues.

Use AVStream.time_base to convert the decoder_seek_where() value, and
pass the current stream number to av_seek_frame().

Use flag AV_TIME_BASE.

Code simplification.

Reduce heap usage by reducing the number of malloc() / free() calls.

The global data structures are now lock-free, because they are
accessed only from the I/O thread.  By using per-request locks, we
have finer grained locking, preparing for locks shared with the
client.

This adds some overheads for indirect calls to the I/O thread, but
reduces the amount of global locks.  Next step will be switching to
per-request locks.

Move the assignment to _request_done(), right before the GCond is
signalled.  This might fix spurious wakeups.

This is a trivial function.  Merge some duplicate code, e.g. the
g_cond_broadcast() call.