Trying to get to the same order of alteredq's cubemap.

Updated some examples.
CanvasRenderer: Skip render when opacity is 0.

Fix is a bit dirty, but at least it's short ;)

Added refractive mapping and mixing combination mode for the environment map and the underlying material.

This commit has handful of things going on:

- had to sacrifice one light to please ANGLE, making it again up to 4 instead of up to 5 total lights :(

- Basic / Lambert / Phong materials now have new parameters:

    "combine": THREE.Mix | THREE.Multiply [default]

      - tells how to combine environment map with the original material:

         - "Multiply" creates chrome-like metallic look (it's what was already there),
           color of the underlying material is multiplied with the environment map,
           this creates different colored metals

         - "Mix" creates shiny plastic look
            color of the underlying material is lerped with the environment map,
            amount of the contribution from the environment map is controlled
            by another new parameter "reflectivity"

    "reflectivity": <float> [from <0.0, 1.0>]

       - controls mixing of underlying material and environment map for "Mix" combine mode
          0: pure underlying material, zero contribution from environment map (Basic/Lambert/Phong)
          1: pure environment map, zero contribution from underlying material (uncolored chrome look)

    "refraction_ratio": <float>

      - controls refractive mapping, ratio of the index of refraction in the medium containing the incident ray
        to that of the medium being entered
      - sensible values seem to be from ~0.6 - 1.0
        (in theory, e.g. air/glass should be 0.75, but this looks weird, values between 0.9 - 1.0 give the most interesting results,
         though maybe there is bug somewhere in my code)

      - for use refraction mapping, just add parameter to TextureCube like this (default is reflection mapping)

          "new THREE.TextureCube( images, THREE.RefractionMap )"

MeshUtils.merge( object1, object2 [ geometry | mesh ] ): Merges object2 into object1. Implemented in materials_cubemap_sky.

CanvasRenderer code clean up

Cleaned up the escher cubemap code a bit. Also, I think it looks much nicer without illumination... ?
At first I though the cube around the sphere was x-inverted (unless the in tention is to simulate refraction...) I've tried changing that but the effect is still weird. I guess those textures work mainly for the sphere and not for the panorama.

I reused the code and did another example using the skymap. More to come ;)

I can see some lines at the edges of the planes though. I guess that's antialiasing kicking in. Should be fixed with a proper cube mesh I guess.

CanvasRenderer has a property called `autoClear` which is true by default. By making it false you have control of when the renderer cleans the screen (by calling renderer.clear()). I wonder if something like that can be added to the WebGLRenderer? That way we can render the cube outside the spheres from a diferent camera (centered in the middle of the screen) than the spheres floating.

Also, added `shading` parameter to `MeshBasicMaterial` (environment mapping was being rendered faceted when using `MeshBasicMaterial` because `shading` was missing (and thus Faceted)).

Also refactored a bit texture cube code, to make it easier to reuse.

TODO:

  - maybe image array loader should go into TextureCube.js so that people could just reuse the code

  - it may be worth to be able to specify different combination of environment map and underlying material (in single render pass),
    currently color/color map/environment map are multiplied, though addition also produces very interesting looking materials

Cloned materials.html -> materials_gl.html to have easier test for WebGLRenderer.

TODO:

Investigate why this test is so slow. It shouldn't be, materials_shaders.html is more complex and yet it is much faster (36 vs 60 fps).

MeshFaceMaterial with every face different is pretty bad use case for WebGLRenderer, but this still doesn't fully explain slowdown :(.

Hooray. So long materials branch, you served us well ;)

Pushed REVISION to 29.

GouraudShading and PhongShading ⟶ SmoothShading