Correct order of exported distance measurements

In commit 50969a2, I changed the API I used to get distance data from
the vision sensors.  Somewhat disappointingly, the new API returns data
in the reverse order from the old API; this commit corrects the order in
Lua before passing it off to C++.
Position sensor higher on vehicle

The sensor was having difficulty reading far-away objects because its
readings would collide with the ground.

Reported-By: Constantin Berzan <cberzan@gmail.com>
Reported-Reference: <CALYHk4UkCX4og+xULtrokM86+tH_=necKG1imH0a5HH_nP0Nuw@mail.gmail.com>
Only track horizontal vehicle motion

At the start of simulation, the vehicle is actually hovering slightly
above the platform; this prevents the simulator from erroneously
inferring initial conditions under which the car intersects the
platform.  This does, however, mean that the car has to fall a couple
inches onto the platform when the simulation begins, so if you record
the absolute speed of the car, you’ll get a weird spike at the start of
the simulation.  This commit fixes that issue by confining speed
measurements to the xy plane.