trackpad-fw: Eliminate need to ditch initial 4 readings of each touch event

Instead of skipping first 4 readings, which incurs a slight latency
penalty at the start of a touch event, we clear lastx/lasty values as
touch events end, and check them before computing deltas. This way, at
the start of every touch event, we only lose one potential delta, and
then start tracking right away after that.
It should be noted, that manual delta computation is only used for
multi-finger gesture tracking at the moment, as the Azoteq sensor
provides smooth relative delta values for us in the case that only one
finger is active.