pointer motion. The derivative estimate is now based on linear
regression from the last four x/y values instead of the last three.
This means that white noise is amplified by a factor of sqrt(.2)
instead of sqrt(.5), ie the factor of improvement is 1.58. Similar
improvements have been observed in data from a real synaptics
touchpad.
Also, the computations are performed in floating point math to avoid
round off errors for very slow finger movements.