This function expected distances per-frame, not per-time which gives us
different behaviors depending on the hardware scanout rate. Fix this by
normalizing to a 12ms frame rate which reflects the touchpad I measured all
the existing thresholds on.

This is a bit of a problem for the test suite which doesn't use proper
intervals and the change to do so is rather invasive. So for now we set the
interval for test devices to whatever the time delta is so we can test the
jumps without having to worry about intervals.

Fixes #121



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