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
GtkKineticScrolling implements the actual physics laws for friction
and springs. When created, position/velocity/boundaries/constants are
given, so at every gtk_kinetic_scrolling_tick() it returns the current
position, and whether the system is in rest.
https://bugzilla.gnome.org/show_bug.cgi?id=729608
