[rex-remind101/MZFormSheetController] fixed rotations for `backgroundImageView`.

Currently, the rotations are only working on the `backgroundImageView` if the user is using `backgroundBlurEffect == YES`. However, the rotations are incorrect if the user explicitly sets `[MZFormSheetController sharedBackgroundWindow].backgroundImage` to their own generated `UIImage`. The following correction works in all cases including iOS < 8 and iOS 8.