Fixes #5570

This fixes colada keyframe example but breaks helpers. Time to refactor Helpers...

The object returnen by the scene's clone method is now a fully valid scene and
can be used in rendering.

Lights with a target will have their target automatically added to the scene to honor matrixAutoUpdate.

I had to resurrect parts of old code (keeping track of scene graph changes).

Good news: performance is great, new scene graph gains compound with buffers optimizations and non-array materials refactoring. Together we gained about 15% for rotating cubes performance test (pushing max to 2650 cubes at 60 fps).

Still broken with new scene graph: lod, skinning, collisions, AnaglyphWebGLRenderer, ParallaxBarrierWebGLRenderer, multi-canvas circle example.

Maybe a bit too early, but just want to see that code clean and simple...

* Added `scene.ambientColor` and `Scene` now ignores `AmbientLights` (transferring hex for temporal backwards compatibility)
* CanvasRenderer and SVGRenderer using `scene.ambientColor`. Tried to update `WebGLRenderer` too, but the method in question doesn't seem to have access to the scene :S
* Removed `scene.origin` and updated examples that used it.
* Improved subdivision example

I could have just used `lookAt( scene.position )`, but eventually I would like to have Scene not extending Object3D.

Updated misc_sound example with same functionality.

Just a single callback is now passed  to loader.load( "scene.js", callbackFinal ) - callback which is called when everything from the scene was already loaded.

If you want to use also callbacks for initialization of procedural synchronously created elements, you can set property of loader:

loader.callbackSync = function( result ) { ... }

If you want to get callback for load progress, set:

loader.callbackProgress = function( progress, result ) { ... }

Also added per scene creation of collision systems (it used to be singleton).

Sound3D ⟶ Sound

Thanks to @mikaelemtinger ;)