Since asking for ARGB by default is still somewhat experimental on X11
and not every toolkit or complex widgets (like WebKit) still do not like
dealing with ARGB visuals, we should switch back to RGB by default - now
that at least we know it works.

For applications (and toolkit integration libraries) that want to enable
the ClutterStage:use-alpha property there is a new function:

  void clutter_x11_set_use_argb_visual (gboolean use_argb);

which needs to be called before clutter_init().

The CLUTTER_DISABLE_ARGB_VISUAL environment variable can still be used
to force this value off at run-time.