aggregator: Set initial position on first buffer
aggregator: Set initial position on first buffer

Set the initial position on the first buffer, otherwise the queue
will grow without limits before the output thread is started.

https://bugzilla.gnome.org/show_bug.cgi?id=781673

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer