mediafile: Load extension at startup with GTK_MEDIA

When the GTK_MEDIA env var is set, check at startup that it works, not
only when the first MediaFile is instantiated.

This has the fortunate side effect that it prints help output for
GTK_MEDIA=help at startup, too.

gtk/gtkmediafile.c

@@ -643,4 +643,8 @@ gtk_media_file_extension_init (void)
                    g_type_name (g_io_extension_get_type (ext)));
         }
     }
+
+  /* If the env var is given, check at startup that things actually work */
+  if (g_getenv ("GTK_MEDIA"))
+    gtk_media_file_get_extension ();
 }