SSAO demo

TransformControls: Simplified mode switching / visibility.

Fixes TransformControl breakage.

Convert two uses of raycaster to using setFromCamera(). One touches
TransformControls but should not affect #6795.

Also fix an error in webgl_interactive_draggablecubes.html when the
cursor is dragged outside of the backing "plane" (which is a large
planar mesh, not an infinite plane), returning no intersections. Now,
when the limit of the bounded plane is reached, the cubes just stop
following the cursor but no error is produced.

Fix tested with webgl_interactive_draggablecubes.html and the editor.

Async callback for cached resources.

BufferAttribute.length not compatible with r71

The definition of `BufferAttribute.length` has changed since r71 (see
#6473).

![r71](https://github.com/mrdoob/three.js/blob/master/src/core/BufferAttribute.js#L18-L22)

![r72dev](https://github.com/mrdoob/three.js/blob/dev/src/core/BufferAttribute.js#L18-L31)

Backward compatibility with r71 should be kept, otherwise, the method
should just be removed!

Implementation should be

```
get length () {

console.warn( 'THREE.BufferAttribute: .length has been deprecated.
Please use .count.' );
return this.array.length;

}
```

For consistency, the `load` call should always be asynchronous, event
when the resource is readily available in cache.

Add unit tests (unittests_three.html only).
Update QUnit to latest version.

Fixes #6779. Does NOT fix #6572.

Let object loader respect its .texturePath property.