Terminal: Enable double-buffering for the terminal widget

This was disabled originally because of performance paranoia, but it
resulted in flickering sometimes, so let's err on the side of nicer
looking terminals. :^)

Userland/Applications/Terminal/main.cpp

@@ -292,7 +292,6 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     auto window = TRY(GUI::Window::try_create());
     window->set_title("Terminal");
-    window->set_double_buffering_enabled(false);
     window->set_obey_widget_min_size(false);
 
     auto terminal = TRY(window->try_set_main_widget<VT::TerminalWidget>(ptm_fd, true));