Cleaned up shader snippets in WebGLRenderer2.

Depth material finally works also in ANGLE.

Instant performance boost in all cube mapping demos ;)

With panoramas there were insane amounts of unnecessary computations done in fragment shader - basically every pixel on the screen was computing the whole ubershader - ouch ouch ouch!

This was the lowest hanging fruit, still some more performance can be gained by removing other stuff from ubershader.

Big thanks to mrdoob for starting to question sanity of ubershader ;)

Made 0xffffff the default color on all materials (0xeeeeee was making the AO a bit darker).

WebGLRenderer optimisation: flattening camera matrices just once per render, it was doing it once per object before.
WebGLRenderer2: Trying to re-create WebGLRenderer from scratch... Creating a program per material instead of just one for everything. Seems to be 3x faster by now...

Added vec3 and color parameters for MeshShaderMaterial. Also moved normal and view matrices into default parameters.

Added WebGL Texture Filters.

Also yet another fix for car demo load progress.

Trying to catch a bug with SmoothShading material objects with no vertexNormals.

`RefractionMap` ⟶ `RefractionMapping`

CanvasRenderer/SVGRenderer: RenderableFace4 is now gone. Unfortunately it created more problems than it solved.
CanvasRenderer now with ShadingSmooth, or something along the lines...

Fixed SVGRenderer.

Wrapping options follow WebGL specs: ClampToEdge [default], Repeat, MirroredRepeat.

Made up MeshCubeMaterial (missing from alteredq branch)

Added MeshCubeMaterial: WebGLRenderer specific material for more efficient rendering of panoramas when doing cube mapping.

Also moved loadImageArray utility function from examples into TextureCube.js

To be used like this:

    var urls = [ "px.jpg", "nx.jpg", "py.jpg", "ny.jpg", "pz.jpg", "nz.jpg" ];
    var images = THREE.loadImageArray( urls );
    var textureCube = new THREE.TextureCube( images );

    // create scene

    var objectMaterial = new THREE.MeshBasicMaterial( { color: 0xffffff, env_map: textureCube } )
    createScene( objectMaterial );

    // create panorama

    var size = 100000;
    var panoMaterial = new THREE.MeshCubeMaterial( { env_map: textureCube } );
    var mesh = new THREE.Mesh( new Cube( size, size, size, 1, 1, null, true ), panoMaterial );
    sceneCube.addObject( mesh );

Warning: this functionality is currently changing a lot, expect things to get broken before stabilization.

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

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 )"

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)).

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

GouraudShading and PhongShading ⟶ SmoothShading

This is actually tricky due to multimaterials and vertex normals.

The practical effects of "shading" parameter in WebGLRenderer are currently following:

- if your model has vertex normals, now you can "dumb down" its shading by specifying THREE.FlatShading for MeshLambertMaterial and MeshPhongMaterial
- if your model has only face normals, it will always have just flat shading, no matter which shading you set, no escape from this
- if your model has multiple materials, smooth shading always "wins", meaning if there is at least one smooth shaded material in multimaterial group, all other materials in this group will be also smooth shaded (this is solvable by having multiple normal buffers, but seems wasteful, at least for the moment, till there are no practical use cases)

Currently implemented shading / material combinations:

  - MeshNormalMaterial: FlatShading [default], GouradShading
  - MeshDepthMaterial:  no shading parameter in material, uses own specific shading
  - MeshBasicMaterial: no shading parameter in material, uses own specific shading
  - MeshLambertMaterial: FlatShading, GouraudShading [default]
  - MeshPhongMaterial: FlatShading, PhongShading [default]

SubstractiveBlending ⟶ SubtractiveBlending

Implemented material blendings (THREE.NormalBlending, THREE.AdditiveBlending, THREE.SubtractiveBlending) in CanvasRenderer
Added MeshNormalMaterial (per face)

Cleaned up CanvasRenderer and SVGRenderer code a bit.

Not all features from the new material format are implemented yet, though already now it's more powerful than the old format (you can now use  texture also in Basic and Phong materials).

Also started to retire old model format: removed old OBJ converter and meshes that it generated.

TODO:
 - take into account shading and blending parameters
 - transplant cube map support from experimental material

Optimised Particles projection in CanvasRenderer.

Most of the examples working.