node.rs: Add a Node::with_impl() method, to access the NodeTrait impl out of band
This is a bit sketchy, because we use the downcast-rs crate to actually be able to downcast a our "node_impl: Box<NodeTrait>" field into the concrete implementation type. At first we'll use this in the marker.rs implementation, as markers are not rendered from the normal ::draw() method. Maybe we'll find a way to make this cleaner at some point.
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