More aggressive walk function

[pathunstrom]

The current walk is fine as is, but we need a walk implementation that doesn't care if the child has its own walk method so that the renderer itself can be sure to reach all renderable objects.

We can call the new method render_walk for now (unless someone has a better idea) and it should only care that each level has children. It can use the render_walk method to get the child and grandchildren, but has to work with the case that render_walk isn't implemented.

Part of supporting #577