Add Support For Nested Scroll Snaps

[Beka Westberg]

Problem Statement

Sometimes you want to nest your scroll snaps inside eachother, i.e. have multiple vertical scroll snaps be children of a horizontal scroll snap. This is currently not supported:

Expected Behavior

Optimally: The vertical component of the input would be handled by any vertical scroll snaps, and the horizontal part of the input would be handled by any horizontal scroll snaps.

Also good: If the initial movement will determine where the input is passed, i.e. if the first drag appears to be more vertical that horizontal, all following input will be passed to the vertical scroll snap, and vise versa.

Also good: Each individual moment of input will determine where it is passed, more vertical inputs will be passed to the vertical scroll snap, and more horizontal inputs will be passed to the horizontal scroll snap.

Steps to Reproduce

1. Create a horizontal scroll snap.
2. Create multiple vertical sub-scroll-snaps inside the horizontal scroll snap's content (they will act like items).
3. Add items to the vertical scroll snaps.
4. Drag horizontally on the vertical scroll snaps. Observe how the horizontal scroll snap does not recieve the input.

Additional Information

Cyprus showed interest in this here.

The Unity UI Extensions Project has a manager for this here.