Use abs position when checking if element is visible in viewport (#437)
Use abs position when checking if element is visible in viewport (#437)

`clientHeight` doesn't tell us anything about where the `viewPortElem` is located in the window relative to where `this.elem` is located. Using absolute values ensures that `_debounceScrolledToBottom()` is always called when the `infinity-loader` component is in view.
hhff/ember-infinity,hhff/ember-infinity