Since commit b4f5f1e46174, the effect is eased in at the scroll
view's edges so that it does not appear out of nowhere. However,
the linear easing used is not the best option, as now the effect
appears so late that content near the edges ends up just being
cut off rather than faded out.
So adjust the easing function to have the effect appear faster.
https://bugzilla.gnome.org/show_bug.cgi?id=694327
b394d184cc