This seems to give me roughly the same behaviour as macos does on the default
0 speed setting.

* Default speed is lower than before by around 30% [1]
* Acceleration kicks in much sooner (130mm/s vs 250mm/s before)
* Acceleration kicks in slower at lower speeds, so the change from 130mm/s to
  150mm/s is less than that of 320mm/s to 350mm/s
* The effect of the speed setting is a wide-range constant (de|ac)celeration
  [2], which means:
  * The unaccelerated baseline up until the threshold now changes with the
    speed setting
  * The threshold is now the same for all speeds
  * The range of the speed setting should now easily cover all desired device
    speeds.
  * Acceleration is steeper at higher speeds
* Deceleration was left as-is.

[1] This may or may not fix the jumping pointer issues caused by the previous
high defaults. When you have high default acceleration you move the finger
slower. This slow movement caused some touchpads (mostly seen on Lenovos) to
create pointer jumps. These weren't seen on synaptics previously because of a
combination of higher user finger speed (thus not triggering the bug) or just
not being as obvious (2px jump vs 10 px jump).

[2] The speed setting is actually a curve, the closer you get to 1.0 the more
difference you see between two different values. The curve's points are:
-1/0, 0/1, 1/5, so the resolution is closer for slow speeds. We still have
double resolution on the setting though so you'll find what you want.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>