Ensure that `pageX/Y` reflect the coordinate relative to the origin of the
window at the time of init (i.e. `clientX/Y` plus `scrollX/Y`). We record it
during init, since `scrollX/Y` may change, which could result in the wrong
`pageX/Y` values; see added tuWPT.

Also track the target-relative coordinate during init (still just `(0, 0)` in
the absence of layout support). This doesn't fundamentally change any
behavior, but makes the implementation more future proof, making it safer to
start handling Image Button submitters when constructing the form data set;
see jsdom#3496 where this code was extracted from.