touchpad: restart the motion filter on touch begin

touchpad: restart the motion filter on touch begin

Our motion filter takes the last couple of vectors to calculate speed,
provided the direction stays the same and it is within a certain timeout. It
does not take into account lifting the finger, so the velocity on the first
event is off.

Real-world impact is mainly on scrolling. Before commit 289e4675
	filter: enforce minimum velocity
the first motion on a scroll was accelerated by a factor of 0 and swallowed.
After 289e4675 the motion was calculated based on the timeout and a fraction
of the expected effect. Now the first scroll motion is based on the real
finger motion since setting the finger down and thus feels a bit more
responsive.

It also makes a couple of test cases using litest_assert_scroll() work again
since the miniumum motion is now as expected.

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