[cssom-view] Moving forward on subpixel positioning for offset*/client*/scroll* properties

Following up on the F2F notes...

For offset*: getBoxQuads totally subsumes these properties and avoids the
craziness of offsetParent at the same time. I don't think we need to add
anything more here.

For scroll*: per list feedback, these are the properties that we reallly
want to support subpixel for. Maybe the compat impact of changing just
these properties to float would be acceptable? It wasn't clear to me from
the discussion. Maybe we'll find out soon :-).

For client*: I think the simplest way forward for these properties (and for
scroll*, if we need to) is to add new box types to BoxType that expose an
element's scrollport rect and "scrolled content" rects.

Rob
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