* Shorter (less typo-prone)
* More intuitive (also "aligns" with gl.POINTS)
* Makes a nice API (Mesh, Line, Points).

Removed IndexBufferAttribute. Promoted index attribute to BufferGeometry. Maybe .addIndices() instead?
/ping @benaadams

- Better inheriting of `clone` + `copy` methods where appropriate. This is done by calling `new this.constructor()` in parent classes. `this.constructor` refers to the current instance's constructor, so it should all compose nicely;

- Pass in attributes directly as arguments to constructor when cloning, where appropriate (if arguments pattern is different from parent class). This mean `copy` isn't relied on to set those attributes.

- Change order of `clone` in some files (for better visibility into its relationship with `copy`, and to maintain consistency; can return to previous order if necessary);

- Fix CubeTexture's clone method

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