touchpad: ignore the ALPS jump to 4095/0

touchpad: ignore the ALPS jump to 4095/0

Some ALPS touchpad send the occasional 4095/0 event on slot 1 during
two-finger interaction before snapping back to the actual position of the
finger. There doesn't seem to be a specific heuristic to predict this so let's
hardcode those values. When detected, overwrite the current touch point with
the position of the last point. This will likely cause a small pointer jump
when the finger later moves to the real position but based on #492 this could
be a second later, so all bets are off anyway.

Fixes https://gitlab.freedesktop.org/libinput/libinput/-/issues/492

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