Migrated FrogScroller to unified api

- Cast double to float
- Cast nfloat to float
- Change RectangleF.CGSize to CGRect.Size
- Change UIImage.CGSize to UIImage.Size
- Change type of overridden property from float to nfloat
- Removed casting of CGRect to CGPoint in UIImage.Draw()
- Removed casting of nfloat to CGRect passed to Math.Floor()