Line: Fix index on intersection.

[jonathanzopes]

Fixed #30414.

Description

Revert to assign line segment index on intersection again.

I'm not sure, if the line loop case is correctly handled.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.27 78.29	336.27 78.29	+0 B +0 B
WebGPU	516.75 143.47	516.75 143.47	+0 B +0 B
WebGPU Nodes	516.22 143.36	516.22 143.36	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.25 112.13	465.25 112.13	+0 B +0 B
WebGPU	588.76 159.63	588.76 159.63	+0 B +0 B
WebGPU Nodes	544.14 149.2	544.14 149.2	+0 B +0 B

[gkjohnson]

This looks right, though I haven't tested it (it should be tested with line loop to make sure the index makes sense). One thing to note is that when a line loop is being intersected against it will return the index as end - 1 which represents the implicit final line connecting the first and last vertices and will require special handling by the end user. This is may be worth documenting.

[Mugen87]

I'll go ahead and merge so we have this in r173. The changes look good to me as well.