WebGLRenderer: Removed gl_FragDepth-based logarithmic depth buffer code

[mrdoob]

Related issue: #22017

Description

This PR removes all gl_FragDepth-based logarithmic depth buffer related code.

[mrdoob]

@jbaicoianu what do you think about this? reason in #22017

[gkjohnson]

Is this previous issue related? #17384

It looks like this PR is changing the behavior of log depth buffer to only apply to vertices rather than fragments which I don't believe is correct.

[Mugen87]

Sidenote: If we decide to keep the current implementation, we should document the MSAA restrictions at the doc page of WebGLRenderer.

[sam6321]

Would it be possible maintain both log depth buffer implementations and leave it up to the user to decide which they want to use?

I use the fragment log depth buffer implementation as part of a 3D globe application, and there are quite a lot of rendering artifacts when I swap to the vertex method.

I would suggest using the vertex method by default, with an option to use the fragment method if desired (Basically the option that was described here).

EDIT: I made a PR for this here: #22082

[sam6321]

Just a note, after the alternative PR I made was declined, that I have an application that relies on threejs' current fragment log depth implementation that would break if this PR was merged and that implementation was removed.

Are there any other acceptable alternatives that can keep fragment log depth around?

[Mugen87]

I vote to keep the current logarithmic depth buffer code until a new solution/replacement (reverse depth buffer) is available.

[Mugen87]

As mentioned in #22041 (comment), we can't sacrifice per-pixel depth values since this potentially break applications. Until an alternative is found, let's keep using GL_EXT_frag_depth with its emulated fallback.