Can't assign global clipping planes

[MarvinKubus]

I've been using the WebGLRenderer clippingPlanes for a while and now that I'm updating from an older version to the current version, they have been made readonly and are no longer assignable.

This example uses the renderer.clippingPlanes: https://github.com/mrdoob/three.js/blob/master/examples/webgl_clipping_advanced.html

This commit made them readonly:
c7d06ae#diff-a49a68ba8d61aea7970812d8ea758823574f49c5afc1f30c47e88ab0e54ec59d

If this is intentional, then what whould be the proposed way of assigning them now, or should they not be readonly?

[Methuselah96]

They probably don't need to be readonly, that was probably a side-effect of the webgl_clipping and webgl_clipping_advanced examples using Object.freeze which produces a readonly array.

Note that with the current types you can still set the property (e.g., renderer.clippingPlanes = []), just like in the webgl_clipping_advanced and webgl_clipping examples, you just can't modify the array (e.g., renderer.clippingPlanes.push(plane)).

I've created #880 to make them mutable in the next version of @types/three.

[MarvinKubus]

Thank you for the info and the fast PR!
It was indeed a .push that lead me here.