Fix for fat line pixelation artifacts

[WestLangley]

Fixes #14197... At least I am hopeful it will.

[pailhead]

Lol, we made the same thing at almost the same time #14275
Reminds me of a Silicon Valley episode :)

I think it would be good development practice to announce something like this (since you did post there), i could have not coded this had i known you'd do the same thing :)

I think this is probably more valid, i split the mesh and added another value in the uv attribute. I wasn't familiar with how the uv's are used elsewhere so didn't want to modify the layout.

[pailhead]

It has a lot of checks though:

2x ||
1x ?
4x >
2x if

By adding another dimension, you can do it with bool isCap = vUv.z < 0.5; and two if checks.

https://github.com/pailhead/three.js/blob/8fb7884a10d81106a82568393ce3c9ea5f0ab56a/examples/js/lines/LineMaterial.js#L207-L217

[mrdoob]

Thanks!