New STATE_RENDRED state for SceneJS.Load nodes

A new STATE_RENDERED state introduced for the SceneJS.Load node and its sub-classes.

In this state, the node has been rendered and its loaded subgraph has taken effect. From here, like from STATE_LOADED, it may transition to STATE_FREED in which its reload is pending after the node has had its subgraph destroyed by SceneJS to reclaim memory after not having been rendered for some time.

This is to support chained loading of content files that depend on each other's created subgraphs, ie. where there may be a Load node that must only be rendered when another Load has loaded and rendered. A concrete example is LoadOBJ which may create a subgraph containing a LoadMTL. The LoadMTL will define Symbols containing material nodes, then the subsequent nodes created by the LoadOBJ will contain Instance nodes that reference the Symbols. The Instances must not attempt to reference the Symbols until they have been defined.