Implement TrackBallControls fixes from js version

See Issue #75.

This switches the "screen" variable to use the domElement's dimensions &
location rather than the window but only if domElement != document.

It also removes the "radius" variable in favor of using each dimension's
value (ie width & height)
Merge pull request #90 from markhats/trackball-updates

Implement TrackBallControls fixes from js version