touchpad: improve pointer jump detection

Previously, we had a hard threshold of 20mm per event frame. That is just
about achievable by really fast movements (in which case you don't care too
much about the jumps anyway because you've already hit the edge of the screen).

Sometimes pointer jumps have lower deltas that are achievable even on slower,
more likely motions. Analysis of finger motion has shown that while a delta
>7mm per event is possible, jumping _by_ 7mm between two events is unlikely
and indicates a pointer jump. So let's diff the most recent delta and the
current delta, if it increases by 7mm between two event frames let's say it's
a pointer jump and discard it.

Helps with but does not fully resolve:
https://gitlab.freedesktop.org/libinput/libinput/issues/80
https://gitlab.freedesktop.org/libinput/libinput/issues/36



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