webvttenc: Separate cues with two line breaks instead of one.

See http://dev.w3.org/html5/webvtt/#syntax

https://bugzilla.gnome.org/show_bug.cgi?id=703673

gst/subenc/gstwebvttenc.c

@@ -131,7 +131,7 @@ gst_webvtt_enc_chain (GstPad * pad, GstObject * parent, GstBuffer * buf)
     gst_buffer_unmap (buf, &map_info);
   }
 
-  g_string_append_c (s, '\n');
+  g_string_append (s, "\n\n");
 
   buf_size = s->len;
   new_buffer = gst_buffer_new_wrapped (g_string_free (s, FALSE), buf_size);