OSD: fix rendering on noncomposited desktop

Use same function as in the composited case. gtk_widget_get_allocated_width/height seems to cause a lot of problems in GTK3

plugins/common/msd-osd-window.c

@@ -207,8 +207,7 @@ draw_when_not_composited (GtkWidget *widget, cairo_t *cr)
         int width;
         int height;
 
-        width = gtk_widget_get_allocated_width (widget);
-        height = gtk_widget_get_allocated_width (widget);
+        gtk_window_get_size (GTK_WINDOW (widget), &width, &height);
         context = gtk_widget_get_style_context (widget);
 
         gtk_style_context_set_state (context, GTK_STATE_FLAG_ACTIVE);