Poptip dragresize

Retarget of https://github.com/palantir/plottable/pull/863 from internal branch so we can all work on it easily
well, we want a pure function to return all of this info at once, so there needs to be an interface for the function to return both simultaneously (since it's computed by 1 fn call). we could split it up afterwards but imo its a bit messier that way.