renderer updates (#191)

Big renderer refactor to remove some global state.
- Renderer no longer uses "global scene", but instead takes scene as parameter to functions
- Visibility is exposed, user can generate visibility for arbitrary camera and some exposed parameters
- Big changes in renderer can make user render code in high level interface not compatible with previous versions
- Fixed occlusion culling const correctness

Documentation/ScriptingAPI-Documentation.md

@@ -126,7 +126,7 @@ You can use the Renderer with the following functions, all of which are in the g
 - PutWaterRipple(String imagename, Vector position)
 - PutDecal(Decal decal)
 - PutEnvProbe(Vector pos)
-- ClearWorld()
+- ClearWorld(opt Scene scene) -- Clears the global scene and the associated renderer resources
 - ReloadShaders(opt string path)
 
 ### Sprite