Add video driver selection to settings menu (based uppon idea from webde...

...signer97)
Having Minetest dump random stack data after the array if Irrlicht adds a driver that we don't support isn't acceptable.  This has to do a bounds check.  Or just iterate with `for (short i = 0; i < ARRAYSIZE(drivernames); i++)`.

Also, I mean does Irrlicht document the order of the drivers in the enum list (the drivers' integer equivalents)?  If it's an implementation detail Irrlicht could reorder the enum at any time, breaking this.