Fixed screen mode in portrait doesn't work correctly with iPhone 6+

The fixed screen mode was supposed to round the content scale up to the next power of two. This breaks horribly on the iPhone 6+. Should change it to round to the nearest integer instead. iPhone 6+ is the first device that didn't fall nicely into a 1/2/4x scale unless the dev was using a custom sized view. Should be pretty safe to simply redefine how it works.
