v4l2pool: Deactivate other pool

When importing buffers from a downstream pool, we need to deactivate
that pool to ensure it will be usable again later. Relying on the
refcount to reach zero does not work, since elements like xvimagesink
keeps a reference on their proposed pool.

sys/v4l2/gstv4l2bufferpool.c

@@ -822,6 +822,7 @@ gst_v4l2_buffer_pool_stop (GstBufferPool * bpool)
   }
 
   if (pool->other_pool) {
+    gst_buffer_pool_set_active (pool->other_pool, FALSE);
     gst_object_unref (pool->other_pool);
     pool->other_pool = NULL;
   }