Limit the movement to 20 mm per event

Touchpads are limited by a fixed sampling rate (usually 80Hz). Some finger
changes may happen too fast for this sampling rate, resulting in two distinct
event sequences:
* finger 1 up and finger 2 down in the same EV_SYN frame. Synaptics sees one
  finger down before and after and the changed coordinates
* finger 1 up and finger 2 down _between_ two EV_SYN frames. Synaptics sees one
  touchpoint move from f1 position to f2 position.

That move causes a large cursor jump. The former could be solved (with
difficulty) by adding fake EV_SYN handling after releasing touchpoints but
that won't fix the latter case.

So as a solution for now limit the finger movement to 20mm per event.
Tests on a T440 and an x220 showed that this is just above what a reasonable
finger movement would trigger. If a movement is greater than that limit, reset
it to 0/0.

On devices without resolution, use 0.25 of the touchpad's diagonal instead.

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

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