MetaBackendX11: Don't XResizeWindow behind clutter's back

In the case of a nested Wayland compositor inside an X session,
Clutter is managing the toplevel window size, so don't call
XResizeWindow on it - this will confuse Clutter and get the size
and the hints out of sync on the toplevel window.

https://bugzilla.gnome.org/show_bug.cgi?id=736279
Moving setting of window-scaling-factor to meta_clutter_init()

Setting the scaling factor immediately after calling clutter_init()
avoids creating the stage at one size, then later resizing it to
a different size.

https://bugzilla.gnome.org/show_bug.cgi?id=736279