leaflet: Transition improvements
Implement 'under' child transition animation, use it for 'over' for right and down directions, matching 'over' description.
Edit: Also clip bottom child.
Edit2: Also fix crossfade, leading to cleaner code for 'under', and add the actual 'under' child transition, which is a reverse of 'over'.
(with a CSS snippet to make backgrounds opaque so that they don't overlap)