- Parameterized geometry (e.g. BoxGeometry) should not clone the base
properties of Geometry.
- CubeTexture: Clone is parameterless.

Add more geometries to ObjectLoader.js and pass down missing parameters
to constructors for `CircleGeometry` and `CylinderGeometry`.

Provide a clone method to the specialized geometries in
src/extras/geometry, otherwise:
- the `.parameters` are not copied
- the `.type` of the clone is "Geometry" or "BufferGeometry"
- the cloned object is not an instance of the initial type. It loses its
specialization and becomes a plain `Geometry` or `BufferGeometry`.

Class deriving from `PolyhedronGeometry` defined  `this.parameters` all
over the place. Standardize to defining the `.type` and `.parameters`
after the `PolyhedronGeometry` constructor is called.

Also fix prototype inheritance chain for classes deriving from
`PolyhedronGeometry`.

Clear the target data before pushing new vertices and faces

- WebGLMultipleRenderers: Use Geometry instead of BufferGeometry
- index.html: add "webgl_multiple_renderers" again
- Misc. fixes to please closure compiler (unused var).

@tschw I think this is the same you were trying to do?