gl: Set glimagesink rank to SECONDARY

This is a full-featured video sink now and especially should be
used instead of osxvideosink on OSX if available.

ext/gl/gstopengl.c

@@ -89,7 +89,7 @@ plugin_init (GstPlugin * plugin)
 #endif
 
   if (!gst_element_register (plugin, "glimagesink",
-          GST_RANK_MARGINAL, GST_TYPE_GLIMAGE_SINK)) {
+          GST_RANK_SECONDARY, GST_TYPE_GLIMAGE_SINK)) {
     return FALSE;
   }