Add client-side double buffering of window backing stores.

This prevents flicker and looks rather good. The main downside is that
resizing gets even more sluggish. That's the price we pay for now.

Applications/Terminal/main.cpp

@@ -88,6 +88,7 @@ int main(int argc, char** argv)
     make_shell(ptm_fd);
 
     auto* window = new GWindow;
+    window->set_double_buffering_enabled(false);
     window->set_should_exit_app_on_close(true);
 
     Terminal terminal(ptm_fd);