layout: Take into account the client point for fixed positioned stacking contexts
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In the Gecko case, this style source would be the style context. In the servo
case, it will be always the computed values.
We could optimise this further in the case of stylo (from three FFI calls to
one) if we use an API of the form CalcAndStore(node, new_cv). But that would
imply borrowing the data twice from Servo (we also have borrow_data_unchecked
fwiw, but...).
