Adding a buffer to the end of a GstBufferList is supposed to be a fast
operation, but it was not since the iterator does not advance its
nextpointer when adding buffers and GList does not have a tail pointer.
Using a GQueue to store the buffers makes it easier to add buffers to
the end of the list and this operation will now be much more efficient.

Adding an entire GList of buffers using
gst_buffer_list_iterator_add_list() will however have to iterate over
the list being added to be able to update the tail pointer in the
GQueue.