Clip plane feature

Somewhat related to #57 we should probably consider standardizing a way to define clip planes eventually.

| Name | Type | Description |
|--------|-------|---------------|
| clipPlanes | `ARRAY1D` of `FLOAT32_VEC4` | Each `VEC4` (A, B, C, D) defines a clip equation A*x + B*y + C*z + D = 0. Elements where the left side is positive are visible while elements on the negative side are clipped. |

Additionally the device or renderer would have an integer property "maxClipPlanes" in case there is a limit.

Since both scene level clipping and object level clipping can make sense this parameter could exist almost anywhere in the scene hierarchy as well as on the renderer. These could either be separate features `KHR_RENDERER_CLIP_PLANES`, `KHR_SURFACE_CLIP_PLANES` etc. or some a priori definition that they exist on say the world, surface and volume?

Start by reviewing the relationship to issue #57 and the proposed clipPlanes and maxClipPlanes definitions in this issue. A complete result would resolve where clip planes belong in the scene hierarchy or renderer, whether they need separate features, and how their limits are represented.

Written by the indexing model from the issue text.