discoverer: consider subtitles as raw

Otherwise, discoverer will generated an "inner" codec
where there can be a tranformation (eg, kate -> DVD SPU,
and various ->text/x-pango-markup).

https://bugzilla.gnome.org/show_bug.cgi?id=639055

gst-libs/gst/pbutils/gstdiscoverer.c

@@ -842,6 +842,9 @@ child_is_raw_stream (GstCaps * parent, GstCaps * child)
     return TRUE;
   }
 
+  if (is_subtitle_caps (parent))
+    return TRUE;
+
   return FALSE;
 }