Make Util.getPosition be relative to page

Commit 5108c463 caused a regression
in the case where scrolling is used -- getPosition return position
relative to the viewport, while getEventPosition expected a position
relative to the page.

As per
https://developer.mozilla.org/en-US/docs/Web/API/Element/getBoundingClientRect,
the fix for this is simply to add the `pageXOffset` and `pageYOffset` to
the output of `getBoundingClientRect()`.

Fixes #459.