Always respect tap_max_distance, not just at the end of the gesture. Fixes #406

In combination with an increased tap_max_touchtime the current implementation
causes a lot of accidental taps when scrolling back and forth.

This is a very simple fix which tracks the distance during the MOVE event.
Merge pull request #425 from Prinzhorn/issue_406

Always respect tap_max_distance, not just at the end of the gesture (#406)