discoverer: copy over result and seekable fields when copying a discoverer info

The function gst_discoverer_info_copy doesn't copy the data members seekable
and result of the source GstDiscovererInfo.

In the case of copying a GstDiscovererInfo for later use, the seekbale will be
undefined, which in practice usually will be false, even though the seekable of
the original GstDiscovererInfo is true.

https://bugzilla.gnome.org/show_bug.cgi?id=762710

gst-libs/gst/pbutils/gstdiscoverer-types.c

@@ -410,6 +410,8 @@ gst_discoverer_info_copy (GstDiscovererInfo * ptr)
         stream_map);
   }
   ret->duration = ptr->duration;
+  ret->result = ptr->result;
+  ret->seekable = ptr->seekable;
   if (ptr->misc)
     ret->misc = gst_structure_copy (ptr->misc);