[2020] better triple monitor support

Thanks for bringing this up! The reasons it's wonky at the moment:

• Field of view is horizontal, rather than vertical. For competitive games enforcing a consistent horizontal field of view between players makes sense, but for Unturned I think it makes more sense to allow a wider horizontal field of view for wide screens.
• User interface safe zones are undefined and not configurable yet, but in the future should allow all of the UI to be kept in the center area.

I'll open a card for this on the quality-of-life roadmap list, as it is fairly important.