filter: adjust acceleration curve depending on speed

filter: adjust acceleration curve depending on speed

The acceleration curve consists of four parts, in ascii-art like this:
        _____________
       /
  ____/
 /
/

where the x axis is the speed, y is the acceleration factor.
The first plateau is at the acceleration factor 1 (i.e. unaccelerated
movement), the second plateau is at the max acceleration factor. The threshold
in the code defines where and how long the plateau is.

This patch adjusts the curve based on a [-1, 1] range. For anything below 0,
the plateau is longer (i.e. accel kicks in at a higher speed), the second
incline is flatter (i.e. accel kicks in slower) and the max accel factor is
lower (i.e. maximum speed is slower). For anything above 0, the inverse is
true, acceleration kicks in earlier, harder and is faster in general. So the
default/min/max curves overlaid look something like this:
      ________ max
     | _______ default
    | /  _____ min
  _|_/_/
 /
/

Note that there's a limit to what ascii art can do...

Note that there are additional tweaks we can introduce later, such as
decreaseing the unaccelerated speed of the device (i.e. lowering the first
plateau).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>