Fixes #38926

This patch implements the iOS behavior pointed out by @dkwingsmt at #38926 , which is also consistent with the performance of my settings application on the iPhone.

### iOS behavior (horizontal or vertical drag)

## Algorithm
When dragging: delta(combined) = max(i of n that are positive) delta(i) - max(i of n that are negative) delta(i)
It means that, if two fingers are moving +50 and +10 respectively, it will move +50; if they're moving at +50 and -10 respectively, it will move +40.

~~TODO~~
~~Write some test cases~~