glTF 2.0: New KHR_environments extension

[McNopper]

For PBR and IB lighting, an environment map is needed. The follwoing JSON snippet defines evironments by example:

{
  "environments": [
    {
      "environmentTexture": 0,
      "type": "sphere"
    },
    {
      "environmentTexture": 0,
      "type": "panorama"
    }
  ]
}

"type" defines, the format of the environment texture.

For non-PBR materials, the texture can be used just as the environment. For PBR, the texture needs to be sampled and pore-filtered. It needs to be discussed, if cube maps as textures should be supported. Furthermore, a standard HDR image format has to be selected. Finally, find out a possibility, to provide the pre-sampled/-filtered images as well.

[UX3D-nopper]

Please continue discussion here: https://github.com/KhronosGroup/glTF/pull/1850