finger-scroll: Fix kinetic scrolling
finger-scroll: Fix kinetic scrolling

The previous code smiplification actually broke kinetic scrolling entirely.
Fix and improve the maths involved.

pwithnall/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx