[cssom-view] Algorithmify the elementFromPoint() algo for external use

[tabatkins]

The UI Events spec wants to be able to talk about hit testing in a reasonably accurate way. While CSS's hit testing is currently a massive hand-waved ball of nonsense, that nonsense is at least abstracted into an algorithm that kinda makes sense in the definition of elementFromPoint()/elementsFromPoint().

We should name/dfn/export those algorithms.

// @garykac