Create sensible barriers for the tracking function

[mikesizz]

currently if you attempt to move the mouse cursor outside of the specified container the lookingglass viewport will get stuck on the edge of said container. this is not a desired effect and should instead exit the container to such a point which the viewport is no longer visible.

in the past i achieved this by applying the tracking to the entire document, but this uses unnecessary and large amounts of power and is a performance drain.

some system needs to be devised to allow the viewport to gracefully exit the container.

below is an image showing how it can get "stuck" on the edge of the container due to the mouse cursor exiting.