Bug 1819741: Constrain acc bounds to that of scroll areas for hit testing, r=morgan

This revision adds logic to BoundsWithOffset to ensure that bounds, when
calculated for hit testing, are constrained to the scroll areas that contain
them. This ensures that we don't return an Accessible that's covered by another
element when hit testing due to overflow: scroll situations.

This revision also contains a fix for transforms and scroll: we now apply
scroll before any transform, since transforms operate on scrolled content.

This revision contains tests for both of the above changes.

Differential Revision: https://phabricator.services.mozilla.com/D173392