LineMaterial: fix segment clipping with reverse depth

[CodyJasonBennett]

Description

Fixes software clipping for line segments in LineMaterial which is sensitive to both NDC range and the reversal of near/far values with reverse depth. This needs an example, but I noticed when using Line2 for axis helpers, axes appeared to be inverted.

[WestLangley]

Can you please provide a derivation for your conservative estimate of the near plane in the case of USE_REVERSEDEPTHBUF?

For reference, here is the original PR.

[CodyJasonBennett]

I'm not sure what you're asking, sorry. Is the method unclear, or do you want me to additionally write how to compose a reversed projection matrix? It's notably using the [0, 1] range but reversed, and I wrote the terms inline to show this if you compare to how we create the non-reversed matrix in https://github.com/mrdoob/three.js/blob/dev/src/math/Matrix4.js#L788.

[WestLangley]

I was expecting something like the following so I can compare with my notes: 🙏

1. begin with the formula for the original (perspective) projection matrix
2. show the formula for the reversed projection matrix
3. identify a and b.
4. propose a formula for the z-coordinate of a conservative estimate of the near plane
5. show that the coordinate lies between 0 and - near.

This parallels what I did in the original PR.