For window-manager managed windows, xwl_realize_window is only called for
the window-manager's decoration window and not for the actual client window
on which we should set the _XWAYLAND_RANDR_EMU_MONITOR_RECTS prop.

Usualy this is not a problem since we walk all client windows to update
the property when the resolution is changed through a randr call.

But for apps which first do the randr change and only then create their
window this does not work, and our xwl_output_set_window_randr_emu_props
call in xwl_realize_window is a no-op as that is only called for the wm
decoration window and not for the actual client's window.

This commit fixes this by making ensure_surface_for_window() call
xwl_output_set_window_randr_emu_props on the first and only child of
window-manager managed windows.

Note this also removes the non-functional xwl_output_set_window_randr_emu_props
call from xwl_realize_window, which was intended to do this, but does not
work.

This fixes apps using the ogre3d library always running at the
monitors native resolution.

Acked-by: Olivier Fourdan <ofourdan@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>