Propagate arbitrary integer types as indices

[gdalle]

The idea is that if we propagate integer types carefully, we could store all indices in Int32 whenever that is enough, leading to a 2x reduction of memory footprint.

Caveats:

• Just because an integer type T is able to store the vector of colors doesn't mean it is able to store every vertex.
• As a result, I use separate integer types for indices (often called T1) and colors (often called T2)

Still a draft, don't merge yet because I fear this might make some existing code error. Also needs tests on Int32 indices.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (87fd1b3) to head (a10ec10).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #222   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           15        15           
  Lines         1716      1720    +4     
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+ Hits          1716      1720    +4     

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