IPHONE: Scale input according to content scale factor.

This hopefully fixes input positions in retina devices. The idea is stolen
from QT: https://qt.gitorious.org/qt/qt/source/0726127285413829f58618b5b82fb3e2da0c3a74:src/plugins/platforms/uikit/quikitwindow.mm#L261-296

Complicated way to retrieve scale's return value properly is taken from:
https://stackoverflow.com/questions/3130464 We sadly can't use the cleaner
solution since we don't want to require a newer SDK...