gst: keep track of queued buffers

Pipewire might update buffers that have not been recycled by GStreamer,
because they are still used downstream. There is nothing we can do about
it in the pipewiresrc.

If a buffer is sent downstream more than once, take an additional
reference to make sure that we don't queue a buffer that is still used
and print a warning.

src/gst/gstpipewirepool.h

@@ -56,6 +56,7 @@ struct _GstPipeWirePoolData {
   goffset offset;
   struct pw_buffer *b;
   GstBuffer *buf;
+  gboolean queued;
 };
 
 struct _GstPipeWirePool {