gdk: No need to clear offscreen window when showing

I don't think it's necessary to clear the backing surface when showing
the window, as we're going to repaint it anyway. If it's needed, we can
implement it again using internal APIs, as the public window_clear()
APIs are going away.

gdk/gdkoffscreenwindow.c

@@ -454,8 +454,6 @@ gdk_offscreen_window_show (GdkWindow *window,
   GdkWindowObject *private = (GdkWindowObject *)window;
   GdkRectangle area = { 0, 0, private->width, private->height };
 
-  gdk_window_clear_area (window, 0, 0,
-			 private->width, private->height);
   gdk_window_invalidate_rect (window, &area, FALSE);
 }