Cuchaz authored Jul 30, 2018 and Peter Hutterer committed Jul 31, 2018

On some ELAN tablets we get a coordinate jump in the same frame that we put
the tip down. The existing axis smoothing causes that jump to be somewhere in
the middle between the previous and the next coordinates, causing a small
stroke from the smoothed position to the next. Prevent this by resetting the
history on tip down/up so we always take that coordinate.

Fixes #94



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