Support clipping planes

[lexaknyazev]

Custom clipping planes could be used for more convenient model inspection.

This feature would consist of:

• Six planes forming a rectangular cuboid either axis- or viewer-oriented
• Sliders to control them, each plane should move only along its normal

Rendering implementation would require a relatively simple vertex shader update:

gl_ClipDistance[0] = dot(position, planeXPos);
gl_ClipDistance[1] = dot(position, planeXNeg);
gl_ClipDistance[2] = dot(position, planeYPos);
gl_ClipDistance[3] = dot(position, planeYNeg);
gl_ClipDistance[4] = dot(position, planeZPos);
gl_ClipDistance[5] = dot(position, planeZNeg);

where position is a vertex position in the world (or view) space and each plane* is a vector with plane equation coefficients in the same space.

The feature would require support for the WEBGL_clip_cull_distance extension.

[UX3D-haertl]

Should we implement this feature?
You said in a Khronos call that you already implemented it somewhere and we only need to integrate it.
Otherwise, we can also do it from scratch.

[lexaknyazev]

Yes, I did that some time ago. The renderer part is probably good to go but the UI would need some extra work. I'll open a PR for the renderer soon.