Eliminate 100 points/second buffer in responding to pan gesture.

This change eliminates a race condition when using the pan gesture to close an exposed
under controller. When panning very slowly (e.g. abs(currentVelocityX) < 100 ) to close
the left or right under controller, it was possible to cause a reset of the top view
(e.g. remove under controller's views from the view hierarchy), immediately followed
by an open of the under controller whose view was just removed. So, despite having
a non-nil under controller, the side would be opened to a blank white screen.
Removing the 100 points/second buffer in the currentVelocityX check eliminates
the race condition.