Make it possible to override behavior of offsetWidth/offsetHeight

An HTML element's `offsetWidth` and `offsetHeight` are set with `Object.defineProperty` as getters that always return `0` ([source](https://github.com/assaf/zombie/commit/f206bc3e35f8e569b102497b41eea0392b6fbcc8#diff-0a71f9e0c26d82bb8f0120554760bfb8R78)). As a result, the jQuery `:visible` filter always returns false ([source](https://github.com/jquery/jquery/blob/10399ddcf8a239acc27bdec9231b996b178224d3/src/css/hiddenVisibleSelectors.js)).

This PR makes it so that the behavior can be overridden.