Work on rendering/scene code

- Add new object type, entity
	- Wraps scene node, data blocks, etc into a single struct for use
	  in game management code.
- Add new error codes
- Adjust client code to use new rendering routines
- Still some tweaking to be done, primarily minimizing coupling between
  separate objects. Scene node/object probably need to be redone, since
  the new entity type of object can handle flagging objects for deletion
  as well as build and update aboslute transforms, etc.
- Also need to iron out how per-object data will be managed. It changes
  based on entity type, so it can't easily be managed at the scene node,
  object, or group level, but instead perhaps at the entity level.