tablet: on tip down/up, force the delta to zero

We may get a pointer jump on tip down/up, see #128. For absolute coordinates
we reset the history to avoid smoothing across that jump but deltas still used
to be calculated based on the previous position to the current one. This
can result in a large jump on tip down.

Since the delta is supposed to be useful (and not physically accurate, see the
docs), let's force it to 0/0 on tip down/up to avoid that scenario.

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