Use Native convertRect:, convertPoint: Methods to Handle Nil Cases

[Adlai-Holler]

Resolves #2888 @hannahmbanana

I didn't know this, but convertPoint:toLayer:nil behaves a lot differently from convertPoint:toView:nil and so it's important that we call the appropriate method for the appropriate case.

[garrettmoon]

I'm not positive this is an issue, but CALayer and UIView document different behavior when the view / layer is nil.

CALayer:
"If you specify nil for the layer parameter, this method returns the original point subtracted from the layer's frame's origin."

UIView:
"If view is nil, this method instead converts to window base coordinates."

[Adlai-Holler]

@garrettmoon It's true the behavior changes, but I think it's reasonable for us to do the layer-thing for layer-backed nodes, and the view-thing for view-backed nodes.

[garrettmoon]

One final comment: could you add documentation to these methods which says it mirror's UIView's behavior and not CALayer's?