Document additional limitations of 3D Tiles classification

[ptrgags]

Description

After investigating #11341 and #11223, the result was that these are limitations of shadow-volume based classification that were not well-documented. The two examples had two different issues:

1. OSM buildings is not watertight (most buildings do not have a bottom). The stencil test used for shadow volumes assumes that the mesh is closed (so there are the same number of front and back faces to render, so this causes the coloring to be flipped for some fragments:

2. For the other issue, the problem happens because the tileset being classified has translucent primitives. Since translucent primitives do not update the depth buffer, the stencil test will give incorrect results. This leads to artifacts for some camera views:

See This Sandcastle for an example

translucent building (has artifacts)	opaque building (no artifacts)

This PR simply documents these additional limitations of 3D Tiles classification.

Issue number and link

Fixes #11341 (by documenting the limitations)

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage Documentation-only change.
• I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[ptrgags]

@ggetz could you review?

[ggetz]

Thanks @ptrgags!

[ptrgags]

@ggetz updated!