Created a separate variable for cc normal, independent perturb implementations

Added the option to use a clearcoat normal independent from the (maybe) perturbed geometry normal.

multiscattering via Fdez-Agüera's "A Multiple-Scattering Microfacet
Model for Real-Time Image-based Lighting".

* move is* properties to prototypes and remove unused ones

* rebuild

* merge dev -> less-is

* remove rebuild from PR

* convert to ES modules

* rebuild

* move shaders back into glsl files

* reinstate polyfills

* clear coat implementation.

* add physical variations to examples.

* add example file for physical materials

* adjust clearcoat range to 0-1 as original.

* switch example color to gray from yellow.

* Reintroduce MeshPhysicalMaterial

* Reintroduce MeshPhysicalMaterial

* Simplified conditionals

- 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

Added texture and image export to the Object3D.js. Works with the scene
export inside the editor.

Extended the .toJson() to safe the texture inside the .json file as a
base64 format.

Big images >2048 are converted to jpg with to keep the size smaller.
For others you can still use the alpha channel of pigs.

ambientOcclusion -> ao per @WestLangley feedback here: https://github.com/mrdoob/three.js/pull/6263#issuecomment-83794671 #2