Proof of bug: missing mercatorMatrix3d

[olsen232]

When writing a CustomLayerInterface in a 3D context, the developer has need of a 3D equivalent to the mercatorMatrix, but none is provided by the interface. This showcases the issue caused by the lack of one, and has a basic example of how to fix.

Currently only intended to showcase the issue - will rewrite once I've opened an issue to discuss the proper fix.

Issue filed as https://github.com/maplibre/maplibre-gl-js/issues/3797

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.73%. Comparing base (ee0450c) to head (624e80c). Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3796      +/-   ##
==========================================
+ Coverage   86.64%   86.73%   +0.08%     
==========================================
  Files         240      240              
  Lines       32273    32276       +3     
  Branches     1977     1974       -3     
==========================================
+ Hits        27963    27993      +30     
+ Misses       3376     3347      -29     
- Partials      934      936       +2     

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[HarelM]

@olsen232 can we close this PR?