Active section changes too early in responsive mode

### Description

When using responsive mode, active section will change when next/prev section hits _about_ half the viewport height, causing callbacks to fire too early.

I'd expect the active section to change upon it hitting the top (if scrolling down), or bottom (if scrolling up) of the viewport.

### Link to isolated reproduction
[Fork of your setReponsive demo](https://codepen.io/deepfriedfilth/full/xemOMa)
You'll see I've added a lime background via CSS to `.section.active` and log `destination.index` + `window.pageYOffset` via `afterLoad` callback to pinpoint when active slide changes

### Steps to reproduce it
1. Set responsive mode to true
2. Scroll