Fix for trackball mouse events when page is scrolled

If the page had been scrolled, the page offset was not being taken into account. For example, if the page had been scrolled vertically, vertical trackball rotation was impossible. This commit fixes that issue. Tested when specifying a `domElement` for the constructor; seems to also work (i.e. no regression) when not specifying one. Tested in Chromium 30.0.1599 and Firefox 25.0.