Ignore volume box z values

[ffreyer]

Description

Fixes https://discourse.julialang.org/t/unwanted-clipping-at-corners-of-glmakie-3d-plot/131053

Volumes are drawn in a box, where the backside of the box "renders". If fragments (pixels) of this box end up outside the near .. far range, OpenGL/WebGL will clip them. And if they get clipped, the entire ray trace for that pixel gets clipped.

To avoid this, the pr hard-codes clip space z coordinates to 0, which should always pass the clipping test -w <= z <= w. For volume plots that don't write z values during ray tracing (or turn it off with enable_depth = false) this may change the z ordering between volume plots and other elements. If I'm not mistaken volumes should end up in front of things more often. I don't think that matters much, because the z ordering was already wrong to begin with. If it does, we could use z = w to move them to the far plane or z = -w to move them to the near plane, instead of the center.

• Updated volume plots to use the ray start depth as their depth. This gives better depth ordering than a static z value, but could cause near-plane clipping. So far I haven't noticed any issues in LScene or Axis3 with it.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[MakieBot]

Benchmark Results

SHA: 2829a39d90ac318cdbb92cdf48483ba168a26952

Warning

These results are subject to substantial noise because GitHub's CI runs on shared machines that are not ideally suited for benchmarking.

[ffreyer]

"Depth Shift" test fails in WGLMakie because it now looks more like GLMakie. (note the bottom corner of the volume)

before	after	GLMakie