Add mainmenu settings for video_driver and fullscreen

The settings tab of the main menu lacks two important settings: video_driver (Important for those who want to use shaders, but get an error message when direct3d9 is used) and fullscreen (which uses core.get_screen_info()).

I added them to the menu, but both of them need the user to restart Minetest to take the changes in effect.
You should use `screen.display_width` and `screen.display_height` here, just like you do above.