kineticscrolling: avoid stutter at tail of kinetic deceleration

kineticscrolling: avoid stutter at tail of kinetic deceleration

When decelerating the kinetic scroll, we can get into a position where it
looks like we are stuttering. This happens because the amount we move is
so little that it takes multiple frames to make forward progress by one
pixel.

This prevents that by detecting when we have reached the slow stutter of
the deceleration and simply stops the deceleration phase immediately.

https://bugzilla.gnome.org/show_bug.cgi?id=765493