elm: Fix _elm_widget_onscreen_is for window

elm: Fix _elm_widget_onscreen_is for window

A window is using ecore_evas geometry value for its evas_object geometry value.
The evas_output_viewport x(y) value which is used in _elm_widget_onscreen_is
is always 0. So _elm_widget_onscreen_is could return EINA_FALSE, if ecore_evas
geometry x(y) value is bigger than 0, even though a window object is on screen.
So it is not correct to compare ecore_output_viewport and evas_object geometry
for a window object. Moreover it does not make sense.