Fix scroll regression near scroll-view ends

Fix regression in a7b0cb46

 which caused
it to be hard to start scrolling at the ends of a scroll-view if using
fine grained scrolling events.

Change-Id: I55f3210150b993281545c3ad5a7356d892fa30b5
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>