Clip plane feature

[progschj]

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 Ax + By + 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?