[fullscreen] Compute sticky position? (#21)

In spite of my proposal in #19, should we also recompute `position: sticky` to `absolute`? I guess we should. `sticky` is somehow a mix of `relative` and `fixed`, and it makes the element always in-flow, which doesn't seem to make sense for top layer.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/21