While on the pad, zero out velocity every second

While on the pad, zero out velocity every second

We integrate acceleration to get velocity, but that means sitting on the pad
for a long time can add substantial error to the velocity value. Each
second, take the velocity value from a full second ago and subtract that out
of the current velocity. Once we detect boost, this will stop, which means
that as long as we detect boost within a second, we won't have subtracted
out any "real" velocity.

This keeps the pad velocity hovering around zero, which is pretty useful.

Signed-off-by: Keith Packard <keithp@keithp.com>