Re: [w3c/touch-events] Why don't Touch objects have offsetX and offsetY properties like MouseEvent does? (#62)

Well, in case it gives a bit more background about why I really want this particular feature:

I have a matrix3d-transformed element depicting the screen of a handheld device, with perspective. I want some interaction on that. I want the user to be able to click/tap/swipe the screen and for it to react as you'd expect. In order to determine where on the display the user has pointed, I need to get from screen/page coordinates back to the local coordinates of the transformed element. That's what `offsetX` and `offsetY` give me, but to get `client*` or `page*` or whatever the other ones are (these are all I have currently if I want to get them from touch events) and turn them back to the local coordinates this is hard, at least as far as I can tell. I managed it: my solution involves getting the inverse of the matrix transformation and multiplying this with the screen/page coordinates. But what a mess, and I certainly wouldn't expect front-end developers without a programming or mathematics background to figure out how to do this.

My solution in fact is a little buggy (there's some error in the calculated position, and interestingly it looks like Firefox's implementation of `offset*` may have the same bug as whatever is in my code, while Chrome's does not), and I'd absolutely love to know what I've done wrong. So in case anyone's interested, I have [an open Stack Overflow question here](http://stackoverflow.com/q/36373114/496046) (including a [jsfiddle of the problem](https://jsfiddle.net/gq1vLaxk/5/)), and I also [reported what I think is a bug to Firefox here](https://bugzilla.mozilla.org/show_bug.cgi?id=1261645).

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