Mutable nodes - node objects with setters & getters

In addition to the functional definition style, support references to nodes as objects that can be modified.

The purely declarative functional style might be a bit of a pain to some people.

This is easily done - scene nodes are higher order functions that work conceptually like this:

SceneJS.myNode = function() {

    var foo = arguments[0];
    var bar = arguments[1];

    return function() {

         // Visit function, called on traversal - does something with foo and bar.
    }
}

They can be extended like so:

SceneJS.myNode = function() {

    var foo = arguments[0];
    var bar = arguments[1];

    return {
          visit: function() {

                  // Visit function, called on traversal - does something with foo and bar.
          },

          setFoo(val) {
               foo = val;
          },

          getFoo() {
               return foo;
          }
    }
}

So we can then do this:

var node1 = SceneJS.myNode({foo:"xx"});

var node2 = SceneJS.myOtherNode( node1 );

node1.setFoo(99);

Furthermore, we could also have a base class that provides mutation of child node lists:

node1.addChild( SceneJS.myNode({ ... });

var node3 = node1.getLastChild();

..and so on.