glTF 2.0: New KHR_materials_displacement extension

Regarding displacement, I suggest the following extension. This one is for geometry displacement. If another displacement technique needs to be defined e.g. parallax, a displacementParallax could be defined.

```
{
"materials": [
{
"displacementGeometry": {
"displacementGeometryFactor": 1.0,
"displacementGeometryTexture": 0,
"displacementGeometryOffset": 0.0
}
}
]
}
```

From the texture, the r channel is used. The factor is used to multiply the channel. If no texture is present, the value is take as it is. Finally, the offset is added to the above value.

No files, tests, or entry points are named. Start by reviewing the proposed displacementGeometry JSON and the stated texture-channel, factor, and offset behavior; the work is done when the extension design is agreed and its specification and implementation scope are defined.

Written by the indexing model from the issue text.