Fix "TV Mode" issues

* When the window was large enough for a 3x viewport, the zoom was
capped at 2x, causing the TV mode to not accurately portray the game
screen. This has been fixed by adding a regular 3x zoom mode and capping
the ratio calculation to 3x zoom. At the moment this means that (much)
bigger screens won't get a 4x viewport, but I can fix that later
* There was a mistake in the ratio calculation, the scrollbars
thicknesses were being multiplied instead of included only once