add snapshot method that uses the old -[CALayer renderInContext:] method

@ndonald2 I needed this method of snapshotting because `rz_imageByCapturingView:afterScreenUpdate:` was too slow and didn't even always work when snapshotting subview hierarchies in `UICollectionViewCell`s. 

If you have any suggestions for documentation to reflect this let me know. 

We could also use the NS_AVAILABLE macros for the two methods, if it would add any benefit.