Make the lightbox work with image-deferrer (bug 1030159)

https://bugzilla.mozilla.org/show_bug.cgi?id=1030159

Losing the swipe w/ snap made me sad, so here is my attempt to keep flipsnap, making image deferring work with it.
It works fine, flipsnap does a 3d transform (but doesn't use the z coordinate), but that works fine when we use `getBoundingClientRect()`